arm: K3: Avoid use of MCU_PSRAM0 before SYSFW is loaded

On HS devices the 512b region of reset isolated memory called
MCU_PSRAM0 is firewalled by default. Until SYSFW is loaded we
cannot use this memory. It is only used to store a single value
left at the end of SRAM by ROM that will be needed later. Save
that value to a global variable stored in the .data section.
This section is used as .bss will be cleared between saving
this value and using it.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Andreas Dannenberg <dannenberg@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>