doc: cmd: add documentation for cpu command

Add documentation for the 'cpu' command, taking NXP i.MX 8M Plus
as a example.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/usage/cmd/cpu.rst

+.. SPDX-License-Identifier: GPL-2.0+
+.. Copyright 2024 NXP
+
+.. index::
+   single: cpu (command)
+
+cpu command
+===========
+
+Synopsis
+--------
+
+::
+
+    cpu list
+    cpu detail
+    cpu release <core ID> <addr>
+
+Description
+-----------
+
+The *cpu* command prints information about the CPUs, and release a CPU core
+to a given address to run applications.
+
+
+cpu list
+~~~~~~~~
+
+The 'list' subcommand lists and prints brief information of all the CPU cores,
+the CPU information is provided by vendors' CPU driver.
+
+cpu detail
+~~~~~~~~~~
+
+The 'detail' subcommand prints more details about the CPU cores, including
+CPU ID, core frequency and feature list.
+
+cpu release
+~~~~~~~~~~~
+
+The 'release' subcommand is used to release a CPU core to run a baremetal or
+RTOS applications.
+The parameter <core ID> is the sequence number of the CPU core to release.
+The parameter <addr> is the address to run of the specified core after release.
+
+
+Examples
+--------
+
+cpu list
+~~~~~~~~
+
+This example lists all the CPU cores On i.MX8M Plus EVK:
+::
+
+    u-boot=> cpu list
+      0: cpu@0      NXP i.MX8MP Rev1.1 A53 at 1200 MHz at 31C
+      1: cpu@1      NXP i.MX8MP Rev1.1 A53 at 1200 MHz at 30C
+      2: cpu@2      NXP i.MX8MP Rev1.1 A53 at 1200 MHz at 31C
+      3: cpu@3      NXP i.MX8MP Rev1.1 A53 at 1200 MHz at 31C
+
+cpu detail
+~~~~~~~~~~
+
+This example prints the details of the CPU cores On i.MX8M Plus EVK:
+::
+
+    u-boot=> cpu detail
+      0: cpu@0      NXP i.MX8MP Rev1.1 A53 at 1200 MHz at 31C
+            ID = 0, freq = 1.2 GHz: L1 cache, MMU
+      1: cpu@1      NXP i.MX8MP Rev1.1 A53 at 1200 MHz at 30C
+            ID = 0, freq = 1.2 GHz: L1 cache, MMU
+      2: cpu@2      NXP i.MX8MP Rev1.1 A53 at 1200 MHz at 31C
+            ID = 0, freq = 1.2 GHz: L1 cache, MMU
+      3: cpu@3      NXP i.MX8MP Rev1.1 A53 at 1200 MHz at 31C
+            ID = 0, freq = 1.2 GHz: L1 cache, MMU
+
+cpu release
+~~~~~~~~~~~
+
+This example shows release the LAST CPU core to run a RTOS application, on
+i.MX8M Plus EVK:
+::
+
+     u-boot=> load mmc 1:2 c0000000 /hello_world.bin
+     66008 bytes read in 5 ms (12.6 MiB/s)
+     u-boot=> dcache flush; icache flush
+     u-boot=> cpu release 3 c0000000
+     Released CPU core (mpidr: 0x3) to address 0xc0000000
+
+
+Configuration
+-------------
+
+The cpu command is available if CONFIG_CMD_CPU=y.
+
+Return code
+-----------
+
+The return value $? is set to 0 (true) if the command is successful,
+1 (false) otherwise.

doc/usage/index.rst

@@ -50,6 +50,7 @@ Shell commands
    cmd/coninfo
    cmd/conitrace
    cmd/cp
+   cmd/cpu
    cmd/cyclic
    cmd/dm
    cmd/ebtupdate