cmd: cpu: add release subcommand

Add a new subcommand 'release' to bring up a core to run baremetal
and RTOS applications.

For example on i.MX8M Plus EVK, release the LAST core to run a RTOS
application, passing the sequence number of the CPU core to release,
here it is 3:
    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

    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

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