ARM: dts: sunxi: h3/h5: add r_uart node

There is an additional UART in the PL I/O block.
Add a node and pinmux for it.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220112173327.26317-1-mans@mansr.com