dt-bindings: reset: add NPCM reset controller documentation

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>.