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

Note in the bindings documentation that pclk should be renamed if a clock
domain is used to enable the optional bus clock.

Signed-off-by: Gareth Williams <gareth.williams.jx@renesas.com>
Link: https://lore.kernel.org/r/1568793876-9009-3-git-send-email-gareth.williams.jx@renesas.com


Signed-off-by: Mark Brown <broonie@kernel.org>