env: ti: j721e-evm: Limit scope of rproc env variables used by R5 SPL

env: ti: j721e-evm: Limit scope of rproc env variables used by R5 SPL

The commit 532c81bae665 ("include: configs: j721e_evm: Add env variables
for mcu_r5fss0_core0 & main_r5fss0_core0") added four different new env
variables 'addr_mainr5f0_0load', 'name_mainr5f0_0fw', 'addr_mcur5f0_0load'
and 'name_mcur5f0_0fw' to the generic environment, but these are only
needed and used in R5 SPL for early-booting the MCU R5FSS0 and Main
R5FSS0 Core0 on J721E SoCs.

These are not really needed for A72 U-Boot, so limit the scope of
these variables only to R5 SPL. While at this, also fix the loadaddr
variable values to include the hex prefix like with other such env
variables.

Cc: Keerthy <j-keerthy@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>