mediatek: clean up platform kernel modules

mediatek: clean up platform kernel modules

Remove kmod-sdhci-mtk as the mtk-sd driver is built-in anyway for the
relevant subtargets in order to support mounting rootfs from eMMC or
SD card.

Add kmod-iio-mt6577-auxadc to support reading the raw values from the
auxadc unit used as in-SoC thermal sensor. This driver was previously
built-in, but as thermal itself works well without it there is no use
for it in every day use of a device. Build the module to still allow
access to the raw values for those who need it.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>