arm64: dts: allwinner: h6: Add MMC1 pins

MMC1 is used on some H6 boards we want to support. Typical use is 4-bit
SDIO interface with a WiFi chip. Add pin definitions for this use case.

As this is the only possible configration for mmc1, make it the default
one, too.

Signed-off-by: Ondrej Jirman <megous@megous.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>