dt-bindings: snps,dw-apb-ssi: Add optional clock bindings documentation

Add documentation to the Synopsys SPI dt-bindings to support an
optional interface clock that may be used for register access.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Signed-off-by: Gareth Williams <gareth.williams.jx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>