TI: ARCH_OMAP2PLUS: Enable SPL_STACK_R and provide default value

On ARCH_OMAP2PLUS platforms we know what the DDR layout is going to be,
and that it is safe to use SPL_STACK_R and provide a default value for
it.  select this and re-sync the defconfigs.

Signed-off-by: Tom Rini <trini@konsulko.com>