DT: nvmem: Add NXP LPC18xx EEPROM memory binding documentation

* NXP LPC18xx EEPROM memory NVMEM driver

Required properties:

  - compatible: Should be "nxp,lpc1857-eeprom"

  - reg: Must contain an entry with the physical base address and length

    for each entry in reg-names.

  - reg-names: Must include the following entries.

    - reg: EEPROM registers.

    - mem: EEPROM address space.

  - clocks: Must contain an entry for each entry in clock-names.

  - clock-names: Must include the following entries.

    - eeprom: EEPROM operating clock.

  - resets: Should contain a reference to the reset controller asserting

    the EEPROM in reset.

  - interrupts: Should contain EEPROM interrupt.

Example:

  eeprom: eeprom@4000e000 {

    compatible = "nxp,lpc1857-eeprom";

    reg = <0x4000e000 0x1000>,

          <0x20040000 0x4000>;

    reg-names = "reg", "mem";

    clocks = <&ccu1 CLK_CPU_EEPROM>;

    clock-names = "eeprom";

    resets = <&rgu 27>;

    interrupts = <4>;

  };