riscv: dts: starfive: introduce a common board dtsi for jh7110 based boards

This is to prepare for Milkv Mars board dts support in the following
patch. Let's factored out common part into .dtsi.

Signed-off-by: Jisheng Zhang <[email protected]>
Reviewed-by: Emil Renner Berthing <[email protected]>
Signed-off-by: Conor Dooley <[email protected]>

Author: xhackerustc