spi: introduce SPI_MODE_X_MASK macro

Provide a macro to filter all SPI_MODE_0,1,2,3 mode in one run.

The latest SPI framework will parse the devicetree in following call
sequence: of_register_spi_device() -> of_spi_parse_dt()
So, driver do not need to pars the devicetree and will get prepared
flags in the probe.

On one hand it is good far most drivers. On other hand some drivers need to
filter flags provide by SPI framework and apply know to work flags. This drivers
may use SPI_MODE_X_MASK to filter MODE flags and set own, known flags:
  spi->flags &= ~SPI_MODE_X_MASK;
  spi->flags |= SPI_MODE_0;
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/20201027095724.18654-2-o.rempel@pengutronix.deSigned-off-by: Mark Brown <broonie@kernel.org>

include/linux/spi/spi.h

@@ -171,6 +171,7 @@ struct spi_device {
 #define	SPI_MODE_1	(0|SPI_CPHA)
 #define	SPI_MODE_2	(SPI_CPOL|0)
 #define	SPI_MODE_3	(SPI_CPOL|SPI_CPHA)
+#define	SPI_MODE_X_MASK	(SPI_CPOL|SPI_CPHA)
 #define	SPI_CS_HIGH	0x04			/* chipselect active high? */
 #define	SPI_LSB_FIRST	0x08			/* per-word bits-on-wire */
 #define	SPI_3WIRE	0x10			/* SI/SO signals shared */