ARM: mxs: Let mmc0 pin detect be generic

Instead of assigning SSP1_DETECT inside mmc0_4bit_pins_a, let the board dts
handle this setup.

Not every board will use MX23_PAD_SSP1_DETECT as SSP1 functionality.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>