spi: introduce macros to set bits_per_word_mask

Introduce two macros to make setting up spi_master.bits_per_word_mask
easier, and avoid mistakes like writing BIT(n) instead of BIT(n - 1).

SPI_BPW_MASK is for a single supported value of bits_per_word_mask.

SPI_BPW_RANGE_MASK represents a contiguous set of bit lengths.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>