There is no need to expose SoC choice to user, we already got
the SoC according to the build target. So default "select MX6UL"
for MX6UL_14x14_EVK target.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>