ARM: exynos_defconfig: Enable Marvell WiFi-Ex support

Exynos5420 Peach Pit and Exynos5800 Peach Pi boards have a MMC/SDIO
Marvell WiFi-Ex chip. This patch enables wireless support and the
mwifiex driver.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Kukjin Kim <kgene@kernel.org>