am43xx: Update CONFIG_SPL_TEXT_BASE

With 1.2 silicon this is now the documented starting usable point for
downloading images to (and corrects a problem with peripheral booting
with prior silicon).  Prior silicon is OK using this address as well.

Signed-off-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>