reset: npcm: using syscon instead of device data

Using syscon device tree property instead of device data to handle the
NPCM general control registers.

In case the syscon not found the code still search for nuvoton,npcm750-gcr
to support DTS backward compatibility.

Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>