mmc: Correct the value of MMC_NUM_PHY_PARTITION

eMMC card can support up to 7 physical partitions, including 2 boot,
1 RPMB and 4 GPs. Change MMC_NUM_PHY_PARTITION from 6 to 7, which is
the correct value.

Signed-off-by: Yi Sun <yi.y.sun@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>