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

The Synopsys SSI driver uses a mandatory clock that is not documented,
so detail it in the device tree bindings. Also correct the spelling of
"pins" in the "Optional Properties" section for the driver.

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>