omap: detect the board after DM is available

In order to use DM_I2C, we need to move the board detection after the
early SPL initialization.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Heiko Schocher <hs@denx.de>