configs: am57xx_hs_evm_defconfig: Reserve EMIF memory used by PPA

configs: am57xx_hs_evm_defconfig: Reserve EMIF memory used by PPA

The AM571x SoC has 1 GB DDR space. As part of normal re-location process
U-Boot copies itself to the top of DDR bank. However, on HS devices, the
top 37 MB is used by PPA and is firewalled. This results in an exception
and the boot fails. Set CONFIG_SYS_MEM_TOP_HIDE to reserve the top 38 MB
memory (aligned to 2MB as per page size for ARM32) to fix the boot.

Note: This limitation does not exist for other AM57x devices, but this
config is applied in the common defconfig since adding a separate
defconfig only for AM571x is not justified. Losing 38MB of memory at the
bootloader stage on other devices is acceptable.

Signed-off-by: Beleswar Padhi <b-padhi@ti.com>
Reviewed-by: Andrew Davis <afd@ti.com>