dra7xx: Unify init stack pointer address across boards

dra7xx: Unify init stack pointer address across boards

The DRA7xx EVM board use a slightly different init stack pointer
address compared to the AM57xx EVM board, for no particular reason.

Align the address with the AM57xx EVM board (which matches the usual
offset to the end of SRAM used by other OMAP devices) prior to removing
it as a per-board option in a later commit.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Reviewed-by: Tom Rini <trini@konsulko.com>
Tested-by: Derald D. Woods <woods.technical@gmail.com>