dts: npcx: shi: support two version of shi hardware

The shi module in npcx4 supports the enhanced buffer mode.
Add a new compatible string "nuvoton,npcx-shi-enhanced" for it.
Then the shi driver can determine if it should use the enhanced buffer
mode based on the compatiable string.

Signed-off-by: Jun Lin <CHLin56@nuvoton.com>