spi: add ability to validate xfer->bits_per_word in SPI core

Allow SPI masters to define the set of bits_per_word values they support.
If they do this, then the SPI core will reject transfers that attempt to
use an unsupported bits_per_word value. This eliminates the need for each
SPI driver to implement this checking in most cases.

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