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

Documentation/devicetree/bindings/spi/cadence-quadspi.txt

@@ -5,6 +5,7 @@ Required properties:
 	Generic default - "cdns,qspi-nor".
 	For TI 66AK2G SoC - "ti,k2g-qspi", "cdns,qspi-nor".
 	For TI AM654 SoC  - "ti,am654-ospi", "cdns,qspi-nor".
+	For Intel LGM SoC - "intel,lgm-qspi", "cdns,qspi-nor".
 - reg : Contains two entries, each of which is a tuple consisting of a
 	physical address and length. The first entry is the address and
 	length of the controller register set. The second entry is the