arm64: allwinner: add BananaPi-M64 support

The Banana Pi M64 board is a typical single board computer based on the
Allwinner A64 SoC. Aside from the usual peripherals it features eMMC
storage, which is connected to the 8-bit capable SDHC2 controller.
Also it has a soldered WiFi/Bluetooth chip, so we enable UART1 and SDHC1
as those two interfaces are connected to it.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>