Documentation/bindings: Document the Alpine MSIX driver

Following the addition of the Alpine MSIX driver, this patch adds the
corresponding bindings documentation.
Acked-by: NMarc Zyngier <marc.zyngier@arm.com>
Signed-off-by: NAntoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: NTsahee Zidenberg <tsahee@annapurnalabs.com>
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>

Documentation/devicetree/bindings/interrupt-controller/al,alpine-msix.txt

+Alpine MSIX controller
+
+See arm,gic-v3.txt for SPI and MSI definitions.
+
+Required properties:
+
+- compatible: should be "al,alpine-msix"
+- reg: physical base address and size of the registers
+- interrupt-parent: specifies the parent interrupt controller.
+- interrupt-controller: identifies the node as an interrupt controller
+- msi-controller: identifies the node as an PCI Message Signaled Interrupt
+		  controller
+- al,msi-base-spi: SPI base of the MSI frame
+- al,msi-num-spis: number of SPIs assigned to the MSI frame, relative to SPI0
+
+Example:
+
+msix: msix {
+	compatible = "al,alpine-msix";
+	reg = <0x0 0xfbe00000 0x0 0x100000>;
+	interrupt-parent = <&gic>;
+	interrupt-controller;
+	msi-controller;
+	al,msi-base-spi = <160>;
+	al,msi-num-spis = <160>;
+};