env: ti: j721e-evm: Update rproc_fw_binaries env variable for J7200

The J7200 SoCs have different number of remote processors, but reuse
the same environment settings as the J721E SoCs. The current env
variable rproc_fw_binaries is geared towards J721E SoCs and is
incorrect for J7200 SoCs. Please see the logic originally added in
commit 0b4ab9c9 ("env: ti: j721e-evm: Add support to boot rprocs
including R5Fs and DSPs").

Fix this by defining the DEFAULT_RPROCS macro appropriately using
the corresponding TARGET_EVM Kconfig symbol. This macro is used by
the 'rproc_fw_binaries' env variable in the common remoteproc env
header file k3_rproc.h.

The list of R5F cores to be started before loading and booting the
Linux kernel are as follows, and mainly comprises of the Main R5FSS0
cores in this order:
   Main R5FSS0 (Split) Core0 : 2 /lib/firmware/j7200-main-r5f0_0-fw
   Main R5FSS0 (Split) Core1 : 3 /lib/firmware/j7200-main-r5f0_1-fw

The MCU R5FSS0 is in LockStep mode and is expected to be booted by
R5 SPL, so it is not included in the list. The order of rprocs to
boot cannot be really modified as only the Main R5FSS0 cores are
involved and Core0 has to be booted first always before the
corresponding Core1.

Signed-off-by: Suman Anna <s-anna@ti.com>