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 <jszhang@kernel.org>
Reviewed-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

Author: xhackerustc