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

commit e9ea262b8a05c2096967624f0f5fc43d7b3d7e32 from
git@git.assembla.com:cavium/WindRiver.linux.git

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>
Signed-off-by: Witold Sadowski <wsadowski@marvell.com>
Change-Id: Iefd4e393627909b877a3cd3abb624405a9c64184
Reviewed-on: https://sj1git1.cavium.com/c/IP/SW/kernel/linux/+/67402
Tested-by: sa_ip-sw-jenkins <sa_ip-sw-jenkins@marvell.com>
Reviewed-by: Chandrakala Chavva <cchavva@marvell.com>
Reviewed-by: Sunil Kovvuri Goutham <sgoutham@marvell.com>
Signed-off-by: Ruiqiang Hao <Ruiqiang.Hao@windriver.com>