ti_omap5_common: Update SPL start address on secure parts

ti_omap5_common: Update SPL start address on secure parts

Updated the CONFIG_SPL_TEXT_BASE to support secure parts (moving
the start address past secure reserved memory and the size of the
security certificate that precedes the boot image on secure devices).
Updated the related CONFIG_SPL_MAX_SIZE to properly reflect the
internal memory actually available on the various device flavors
(Common minimum internal RAM guaranteed for various flavors of
DRA7xx/AM57xx is 512KB).

Signed-off-by: Daniel Allred <d-allred@ti.com>
Signed-off-by: Madan Srinivas <madans@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>