dt-bindings: reset: add NPCM reset controller documentation

Added device tree binding documentation for Nuvoton BMC
NPCM reset controller.
Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

Documentation/devicetree/bindings/reset/nuvoton,npcm-reset.txt

+Nuvoton NPCM Reset controller
+
+Required properties:
+- compatible : "nuvoton,npcm750-reset" for NPCM7XX BMC
+- reg : specifies physical base address and size of the register.
+- #reset-cells: must be set to 2
+
+Optional property:
+- nuvoton,sw-reset-number - Contains the software reset number to restart the SoC.
+  NPCM7xx contain four software reset that represent numbers 1 to 4.
+
+  If 'nuvoton,sw-reset-number' is not specfied software reset is disabled.
+
+Example:
+	rstc: rstc@f0801000 {
+		compatible = "nuvoton,npcm750-reset";
+		reg = <0xf0801000 0x70>;
+		#reset-cells = <2>;
+		nuvoton,sw-reset-number = <2>;
+	};
+
+Specifying reset lines connected to IP NPCM7XX modules
+======================================================
+example:
+
+        spi0: spi@..... {
+                ...
+                resets = <&rstc NPCM7XX_RESET_IPSRST2 NPCM7XX_RESET_PSPI1>;
+                ...
+        };
+
+The index could be found in <dt-bindings/reset/nuvoton,npcm7xx-reset.h>.