arm: mach-k3: j721e: Move booting of Main R5FSS Core0 to A72 U-Boot

The Main R5FSS Core0 on J721E SoCs is originally booted from R5 SPL
itself to achieve certain product-level early-boot metrics. This is
no longer supported after the R5 SPL re-architecture (support merged
for v2021.10-rc1). Move the booting of this core altogether from R5
SPL to A72 U-Boot.

The env variables are left as is for now, and will be cleaned up
in a subsequent patch.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210726211311.5977-2-s-anna@ti.com