ITE dts/riscv/ite: separate dtsi node to it81xx2 from it8xxx2

The interrupt and watchdog registers of the it82xx2 will be
remapped, so these device nodes should be separated to
it81xx2 from it8xxx2. it8xxx2 dtsi are common settings
for it81xx2 and it82xx2.

Signed-off-by: Ruibin Chang <Ruibin.Chang@ite.com.tw>