mediatek: fix the name of buswidth to bus-width

mediatek: fix the name of buswidth to bus-width

Fix the issue of dts buswidth cannot be applied properly with spi driver.
Fix the name of buswidth to bus-width in dts in order to fit the format
in linux spi kernel[1] so that spi-tx-bus-width & spi-rx-bus-width can be
parsed properly.

[1] Documentation/devicetree/bindings/spi/spi-controller.yaml

Signed-off-by: Chen Minqiang <ptpt52@gmail.com>