i2c: npcm: Change the way of getting GCR regmap

Change the way of getting NPCM system manager reigster (GCR)
and still maintain the old mechanism as a fallback if getting
nuvoton,sys-mgr fails while working with the legacy devicetree
file.

Signed-off-by: Tali Perry <tali.perry1@gmail.com>
Signed-off-by: Tyrone Ting <kfting@nuvoton.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>