dts: sun8i-h3: add pinmux definitions for UART2-3

These are the pinmux definitions for UART2-3 on H3. These UARTs can only
be muxed to these pins, so _a and @0 do not really make sense. I have
left out RTS/CTS, since these are rarely used. These can easily be
enabled using an additional pinmux set.

Signed-off-by: Jorik Jonker <jorik@kippendief.biz>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>