mtd: st_spi_fsm: Add device-tree binding documentation

Cc: [e-mail]
Acked-by Angus Clark <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>
Signed-off-by: Brian Norris <[e-mail]>

Documentation/devicetree/bindings/mtd/st-fsm.txt

···
+* ST-Microelectronics SPI FSM Serial (NOR) Flash Controller
+
+Required properties:
+  - compatible : Should be "st,spi-fsm"
+  - reg        : Contains register's location and length.
+  - reg-names  : Should contain the reg names "spi-fsm"
+  - interrupts : The interrupt number
+  - pinctrl-0  : Standard Pinctrl phandle (see: pinctrl/pinctrl-bindings.txt)
+
+Optional properties:
+  - st,syscfg          : Phandle to boot-device system configuration registers
+  - st,boot-device-reg : Address of the aforementioned boot-device register(s)
+  - st,boot-device-spi : Expected boot-device value if booted via this device
+
+Example:
+	spifsm: spifsm@fe902000{
+	        compatible         = "st,spi-fsm";
+	        reg                =  <0xfe902000 0x1000>;
+	        reg-names          = "spi-fsm";
+	        pinctrl-0          = <&pinctrl_fsm>;
+		st,syscfg	   = <&syscfg_rear>;
+	        st,boot-device-reg = <0x958>;
+	        st,boot-device-spi = <0x1a>;
+		status = "okay";
+	};
+