ARM: multi_v7_defconfig: re-enable SDHCI drivers

Following 5d01b768 "mmc: simplify SDHCI Kconfig dependencies",
SDHCI drivers that use MMC_SDHCI_PLTFM no longer select it, but
instead depend on it. This means that multi_v7_defconfig no longer
selects it, and hence many SDHCI drivers are no longer enabled.
Explicitly enable MMC_SDHCI_PLTFM to solve this.

Fixes: 5d01b768 ("mmc: simplify SDHCI Kconfig dependencies")
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Matt Porter <mporter@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>