igep00x0: default mmcrootfstype to EXT4

omap2plus_defconfig now has built-in compile support for EXT4 enabled
by default now. So, we can use EXT4 as the default root file system
type for MMC.

Signed-off-by: Javier Martinez Canillas <javier@dowhile0.org>
Acked-by: Enric Balletbo i Serra <eballetbo@gmail.com>