spi: spi-mtk-nor: add new clock name 'axi' for spi nor

spi: spi-mtk-nor: add new clock name 'axi' for spi nor

Some mtk spi nor has dedicated dma(s) inside. Add a new clock name, axi,
for spi nor dma bus clock.

Signed-off-by: Tinghan Shen <tinghan.shen@mediatek.com>
Link: https://lore.kernel.org/r/20211220121825.6446-3-tinghan.shen@mediatek.com
Signed-off-by: Mark Brown <broonie@kernel.org>