am43xx: Define common init stack pointer address in Kconfig

The init stack pointer was defined to the same value in each AM43xx
board defconfig. Move the common definition to the Kconfig option
declaration instead.

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