arm64: dts: allwinner: a64: NanoPi-A64: Add Wifi chip

The NanoPi-A64 has an on-board WiFi chip, connected to the usual MMC1 SDIO
interface. The AXP power line is the always-on VDD_SYS_3.3V, but it uses
pin L2 to enable the regulator.
As the actual WiFi driver is not in mainline Linux, it doesn't have a
compatible string, so we omit this from the node.

Add the respective nodes to the DT to make it usable.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
[wens@csie.org: Add RTL8189ETV LPO clock to pwrseq node]
Signed-off-by: Chen-Yu Tsai <wens@csie.org>