spi: spi-fsl-dspi: Make bus-num property optional

The SPI bus number is completely optional to Linux, so make the
corresponding device tree property optional as well.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Link: https://lore.kernel.org/r/20200305115546.31814-1-s.hauer@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>