spi/spi-pxa2xx: add PXA2xx SSP SPI Controller

This is the SPI controller found on Marvel MMP2 and perhaps more
platforms.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-pxa2xx.txt

+PXA2xx SSP SPI Controller
+
+Required properties:
+- compatible: Must be "marvell,mmp2-ssp".
+- reg: Offset and length of the device's register set.
+- interrupts: Should be the interrupt number.
+- clocks: Should contain a single entry describing the clock input.
+- #address-cells:  Number of cells required to define a chip select address.
+- #size-cells: Should be zero.
+
+Optional properties:
+- cs-gpios: list of GPIO chip selects. See the SPI bus bindings,
+  Documentation/devicetree/bindings/spi/spi-bus.txt
+
+Child nodes represent devices on the SPI bus
+  See ../spi/spi-bus.txt
+
+Example:
+	ssp1: spi@d4035000 {
+		compatible = "marvell,mmp2-ssp";
+		reg = <0xd4035000 0x1000>;
+		clocks = <&soc_clocks MMP2_CLK_SSP0>;
+		interrupts = <0>;
+	};