TI:OMAP4: enable EXT support in SPL for OMAP4 boards

Enable EXT support in SPL for OMAP4 boards.
Build tested for duovero, omap4_sdp4430 and omap4_panda.
Run time tested on omap4_panda.

Signed-off-by: Guillaume GARDET <guillaume.gardet@free.fr>
Cc: Tom Rini <trini@ti.com>