arm64: dts: Add Actions Semi S900 and Bubblegum-96

Add Device Trees for Actions Semiconductor S900 SoC and
uCRobotics Bubblegum-96 board.

UART0/1/4/6 interrupts are guesses.

Cc: 96boards@ucrobotics.com
Signed-off-by: Andreas Färber <afaerber@suse.de>