mmc: Add hardware dependencies for sdhci-pxav3 and sdhci-pxav2

I seem to understand that the sdhci-pxav3 and sdhci-pxav2 drivers are
only needed on the MMP architecture. So add a hardware dependency on
ARCH_MMP, so that other users don't get to build useless drivers.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Chris Ball <chris@printf.net>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Eric Miao <eric.y.miao@gmail.com>
Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>