arm64: dts: allwinner: h6: tanix-tx6: Add PIO power supplies

While there is no publicly available schematic of this board, it's not
hard to determine voltage of GPIO port C, D and G (only ones which can
be set).

Port C and G are used for MMC/SDIO communication, so they use 1.8 V
power supply. It's not clear if port D is even used, but if it is, it's
pretty safe to assume it uses 3.3 V power supply. Value read from PIO
Group Withstand Voltage Mode Select register from within pre-installed
Android agrees with that assesment.

Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20210722161220.51181-3-jernej.skrabec@gmail.com