spi: pl022: Document property values

The property "pl022,com-mode" can only assume one of the values of
the enum ssp_mode, defined in include/linux/amba/pl022.h
List the possible numeric values and report the associated meaning.

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>