spi: spi-mtk-nor: add axi clock control for MT8192 spi-nor

spi: spi-mtk-nor: add axi clock control for MT8192 spi-nor

MT8192 spi-nor is an independent sub system, we need extra control axi
bus clock for it. Add support for the additional axi clock to allow it
to be configured appropriately.

Signed-off-by: bayi cheng <bayi.cheng@mediatek.com>
Tested-by: Ikjoon Jang <ikjn@chromium.org>
Link: https://lore.kernel.org/r/1605084902-13151-2-git-send-email-bayi.cheng@mediatek.com
Signed-off-by: Mark Brown <broonie@kernel.org>