dts: pcc: npcx: add ram-pd-depth property in npcx-pcc node

Since the valid bit-depth of RAM_PDn registers are different, this CL
introduces a 'ram-pd-depth' property in 'nuvoton,npcx-pcc' node if the
application needs to turn off the partial ram blocks for better power
consumption.

Signed-off-by: Mulin Chao <mlchao@nuvoton.com>