ARM: mx31pdk: Fix build by passing IMX_HAVE_PLATFORM_MXC_MMC

mx31pdk target fails to build:

  CC      init/version.o
  LD      init/built-in.o
  LD      .tmp_vmlinux1
arch/arm/mach-imx/built-in.o: In function `mx31_3ds_init':
mach-mx31_3ds.c:(.init.text+0x548): undefined reference to `imx_add_mxc_mmc'
mach-mx31_3ds.c:(.init.text+0x69c): undefined reference to `imx31_mxc_mmc_data'
make: *** [.tmp_vmlinux1] Error

Fix this by passsing IMX_HAVE_PLATFORM_MXC_MMC option.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>