dt-bindings: phy: intel-emmc-phy: Add YAML schema for LGM eMMC PHY

Add a YAML schema to use the host controller driver with the
eMMC PHY on Intel's Lightning Mountain SoC.

Signed-off-by: Ramuthevar Vadivel Murugan <vadivel.muruganx.ramuthevar@linux.intel.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>