riscv: dts: spacemit: add UART pinctrl combinations

riscv: dts: spacemit: add UART pinctrl combinations

Add UART pinctrl configurations based on the SoC datasheet and the
downstream Bianbu Linux tree. The drive strength values were taken from
the downstream implementation, which uses medium drive strength.
CTS/RTS are moved to separate *-cts-rts-cfg states so boards can enable
hardware flow control conditionally.

Signed-off-by: Hendrik Hamerlinck <hendrik.hamerlinck@hammernet.be>
Reviewed-by: Yixun Lan <dlan@gentoo.org>
Reviewed-by: Troy Mitchell <troy.mitchell@linux.spacemit.com>
Link: https://lore.kernel.org/r/20250917065907.160615-1-hendrik.hamerlinck@hammernet.be
Signed-off-by: Yixun Lan <dlan@gentoo.org>