ARM: mach-imx: Kconfig: Do not select Babbage for MACH_IMX51_DT

When mx51 dt support was added it was necessary to select MACH_MX51_BABBAGE in
order to get the mx51 dt built without errors.

At that time, there was no pinctrl support for mx51 yet.

Currently there is no need to select MACH_MX51_BABBAGE anymore.

Reported-by: Matt Sealy <matt@genesi-usa.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>