dt-bindings: riscv: add SpacemiT K1 bindings

dt-bindings: riscv: add SpacemiT K1 bindings

Add DT binding documentation for the SpacemiT K1 SoC[1] and the Banana
Pi BPi-F3 board[2] which used it.

Link: https://www.spacemit.com/en/spacemit-key-stone-2/
Link: https://docs.banana-pi.org/en/BPI-F3/BananaPi_BPI-F3
Signed-off-by: Yangyu Chen <cyy@cyyself.name>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Matthias Brugger <matthias.bgg@kernel.org>
Signed-off-by: Yixun Lan <dlan@gentoo.org>