davinci: omapl138_lcdk: add NAND SPL boot support

NAND SPL boot was missing. Add it. The README specific to omapl138-lcdk
is also removed because its content does not apply anymore, i.e. the
generated AIS image can be flashed directly to the NAND without
using any external tool to create and bootable AIS image.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Reviewed-by: Tom Rini <trini@konsulko.com>