ARM: mvebu: re-enable SDHCI driver for Armada 38x SoC in v7 defconfig

In the recent update of mvebu_v7_defconfig a config that enables sdhci-pxav3
driver, that supports SDHCI interface of Armada 38x SoC, disappeared.

This commit enables CONFIG_MMC_SDHCI_PXAV3 back.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Reviewed-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Fixes fc9fa8714a75 ("ARM: mvebu: update v7 defconfig with useful options")
Link: https://lkml.kernel.org/r/1415980652-7429-2-git-send-email-mw@semihalf.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>