am33xx: Unify init stack pointer address across boards

am33xx: Unify init stack pointer address across boards

The AM33xx chiliboard and guardian boards use a slightly different
init stack pointer address compared to other AM33xx boards, for no
particular reason.

Align the address with other AM33xx boards 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>