ARM: imx6q_logic: With SPL_OF_CONTROL enabled, remove MMC init

Since the board uses SPL_OF_CONTROL now, we don't need to
explicitly initialize the MMC driver, but we still need to
pinmux the corresponding pins.  This patch removes the
initialization code and leave just the muxing behind.

Signed-off-by: Adam Ford <aford173@gmail.com>