spi: spi-mtk-nor: use dma_alloc_coherent() for bounce buffer

Use dma_alloc_coherent() for bounce buffer instead of kmalloc() to
make sure the bounce buffer to be allocated within its DMAable range.

Signed-off-by: Ikjoon Jang <ikjn@chromium.org>
Link: https://lore.kernel.org/r/20201006155010.v5.2.I06cb65401ab5ad63ea30c4788d26633928d80f38@changeid
Signed-off-by: Mark Brown <broonie@kernel.org>