dt-bindings: spi: cadence-qspi: Add support for Intel lgm-qspi

Add new vendor specific compatible string to check Intel's Lightning
Mountain(LGM) QSPI features enablement in cadence-quadspi driver.

Signed-off-by: Ramuthevar Vadivel Murugan <vadivel.muruganx.ramuthevar@linux.intel.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201124041840.31066-6-vadivel.muruganx.ramuthevar@linux.intel.com


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