spi: omap-spi: Add compatible for AM654 SoC

AM654 SoC has same McSPI IP as OMAP2+ platforms. Add new compatible to
support McSPI on AM654 SoC.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/omap-spi.txt

@@ -2,6 +2,7 @@ OMAP2+ McSPI device
 
 Required properties:
 - compatible :
+  - "ti,am654-mcspi" for AM654.
   - "ti,omap2-mcspi" for OMAP2 & OMAP3.
   - "ti,omap4-mcspi" for OMAP4+.
 - ti,spi-num-cs : Number of chipselect supported  by the instance.