ti_omap5_common: mark region of DRAM protected on HS parts

If the ending portion of the DRAM is reserved for secure
world use, then u-boot cannot use this memory for its relocation
purposes. To prevent issues, we mark this memory as PRAM and this
prevents it from being used by u-boot at all.

Signed-off-by: Daniel Allred <d-allred@ti.com>