dts: sun8i-h3: split off RTS/CTS for UART1 in seperate pinmux

This was done to make UART1-3 on H3 consistent, and less complicated to
enable UART1-3 on the breakout header on the several H3 board (notably
Orange Pi's). This patch adds a bit of complexity for the existing Banana
Pi, which already had the RTS/CTS associated on UART1.

The RTS/CTS for UART2-3 could be defined in the same way, but since
there is no actual use case for them at the moment, they are left out.

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