spi: quad: fix the name of DT property in patch

The previous property name spi-tx-nbits and spi-rx-nbits looks not
human-readable. To make it consistent with other devices, using property
name spi-tx-bus-width and spi-rx-bus-width instead of the previous one
specify the number of data wires that spi controller will work in.
Add the specification in spi-bus.txt.
Signed-off-by: Nwangyuhang <wangyuhang2014@gmail.com>
Signed-off-by: NMark Brown <broonie@linaro.org>