EEPROMs (SPI) compatible with Atmel at25.

Required properties:
- compatible : Should be "<vendor>,<type>", and generic value "atmel,at25".
  Example "<vendor>,<type>" values:
    "microchip,25lc040"
    "st,m95m02"
    "st,m95256"

- reg : chip select number
- spi-max-frequency : max spi frequency to use
- pagesize : size of the eeprom page
- size : total eeprom size in bytes
- address-width : number of address bits (one of 8, 9, 16, or 24).
  For 9 bits, the MSB of the address is sent as bit 3 of the instruction
  byte, before the address byte.

Optional properties:
- spi-cpha : SPI shifted clock phase, as per spi-bus bindings.
- spi-cpol : SPI inverse clock polarity, as per spi-bus bindings.
- read-only : this parameter-less property disables writes to the eeprom

Obsolete legacy properties can be used in place of "size", "pagesize",
"address-width", and "read-only":
- at25,byte-len : total eeprom size in bytes
- at25,addr-mode : addr-mode flags, as defined in include/linux/spi/eeprom.h
- at25,page-size : size of the eeprom page

Additional compatible properties are also allowed.

Example:
	eeprom@0 {
		compatible = "st,m95256", "atmel,at25";
		reg = <0>
		spi-max-frequency = <5000000>;
		spi-cpha;
		spi-cpol;

		pagesize = <64>;
		size = <32768>;
		address-width = <16>;
	};
