mailbox: add el3 support only for zynqmp platform

If U-Boot is running in Exception Level 3 then use hardcode
register values for mailbox message passing with PMU.
This is only supported for zynqmp platform.

Signed-off-by: Tanmay Shah <tanmay.shah@amd.com>
Link: https://lore.kernel.org/r/20231204215620.63334-3-tanmay.shah@amd.com


Signed-off-by: Michal Simek <michal.simek@amd.com>