spi: cadence: add dt-bindings documentation for Cadence XSPI controller

Add DT binding for Cadence's XSPI controller driver.

Signed-off-by: Konrad Kociolek <konrad@cadence.com>
Signed-off-by: Jayshri Pawar <jpawar@cadence.com>
Signed-off-by: Parshuram Thombare <pthombar@cadence.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1632038705-23805-1-git-send-email-pthombar@cadence.com
Signed-off-by: Mark Brown <broonie@kernel.org>