arm64: defconfig: Allow Mediatek boards to boot from the mmc

Enable the MMC_MTK option necessary to boot Mediatek boards from the
mmc. Booting from the mmc is the common method used for a bunch of
boards, specially Chromebooks, to boot from. Hence this driver is
built-in.
Signed-off-by: NEnric Balletbo i Serra <enric.balletbo@collabora.com>
Link: https://lore.kernel.org/r/20210423075250.2616275-1-enric.balletbo@collabora.comSigned-off-by: NMatthias Brugger <matthias.bgg@gmail.com>

arch/arm64/configs/defconfig

@@ -865,6 +865,7 @@ CONFIG_MMC_DW_K3=y
 CONFIG_MMC_DW_ROCKCHIP=y
 CONFIG_MMC_SUNXI=y
 CONFIG_MMC_BCM2835=y
+CONFIG_MMC_MTK=y
 CONFIG_MMC_SDHCI_XENON=y
 CONFIG_MMC_SDHCI_AM654=y
 CONFIG_MMC_OWL=y