irqchip: st: Add documentation for STi based syscfg IRQs

Signed-off-by: Lee Jones <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Jason Cooper <[e-mail]>

Documentation/devicetree/bindings/interrupt-controller/st,sti-irq-syscfg.txt

···
+STMicroelectronics STi System Configuration Controlled IRQs
+-----------------------------------------------------------
+
+On STi based systems; External, CTI (Core Sight), PMU (Performance Management),
+and PL310 L2 Cache IRQs are controlled using System Configuration registers.
+This driver is used to unmask them prior to use.
+
+Required properties:
+- compatible	: Should be set to one of:
+			"st,stih415-irq-syscfg"
+			"st,stih416-irq-syscfg"
+			"st,stih407-irq-syscfg"
+			"st,stid127-irq-syscfg"
+- st,syscfg	: Phandle to Cortex-A9 IRQ system config registers
+- st,irq-device	: Array of IRQs to enable - should be 2 in length
+- st,fiq-device	: Array of FIQs to enable - should be 2 in length
+
+Optional properties:
+- st,invert-ext	: External IRQs can be inverted at will.  This property inverts
+		  these IRQs using bitwise logic.  A number of defines have been
+		  provided for convenience:
+			ST_IRQ_SYSCFG_EXT_1_INV
+			ST_IRQ_SYSCFG_EXT_2_INV
+			ST_IRQ_SYSCFG_EXT_3_INV
+Example:
+
+irq-syscfg {
+	compatible    = "st,stih416-irq-syscfg";
+	st,syscfg     = <&syscfg_cpu>;
+	st,irq-device = <ST_IRQ_SYSCFG_PMU_0>,
+			<ST_IRQ_SYSCFG_PMU_1>;
+	st,fiq-device = <ST_IRQ_SYSCFG_DISABLED>,
+			<ST_IRQ_SYSCFG_DISABLED>;
+	st,invert-ext = <(ST_IRQ_SYSCFG_EXT_1_INV | ST_IRQ_SYSCFG_EXT_3_INV)>;
+};