riscv: dts: starfive: Add common DT for JH7100 based boards

In preparation for adding initial device tree support for the StarFive
VisionFive board, which is similar with BeagleV Starlight, move most
of the content from jh7100-beaglev-starlight.dts to a new file, to be
shared between the two boards.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Matthias Brugger <mbrugger@suse.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>