env: ti: k3_rproc: Add common rproc environment variables

env: ti: k3_rproc: Add common rproc environment variables

Add a new file include/environment/ti/k3_rproc.h that defines
common environment variables useful for booting various remote
processors from U-Boot. This file is expected to be included in
the board config files with the EXTRA_ENV_RPROC_SETTINGS added
to CONFIG_EXTRA_ENV_SETTINGS and DEFAULT_RPROCS macro overwritten
to include the actual list of processors to be booted.

The 'boot_rprocs' variable just needs to be added to the board's
bootcmd to automatically boot the processors, and runtime control
can be achieved through the 'dorprocboot' variable.

The variables are currently defined to use MMC as the boot media,
and can be expanded in the future to include other boot media.
The immediate usage is intended for K3 J721E SoCs.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>