ARM: configs: add CONFIG_MMC_SDHCI_PXAV3 to the multi_v7_defconfig

The Marvell Armada 38x platform has a SDHCI interface managed by the
sdhci-pxav3 MMC host driver. It therefore makes sense to enable this
driver in multi_v7_defconfig, which supports the Armada 38x platform.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Olof Johansson <olof@lixom.net>