Binding for ASCII LCD displays on Imagination Technologies boards

Required properties:
- compatible : should be one of:
    "img,boston-lcd"
    "mti,malta-lcd"
    "mti,sead3-lcd"

Required properties for "img,boston-lcd":
- reg : memory region locating the device registers

Required properties for "mti,malta-lcd" or "mti,sead3-lcd":
- regmap: phandle of the system controller containing the LCD registers
- offset: offset in bytes to the LCD registers within the system controller

The layout of the registers & properties of the display are determined
from the compatible string, making this binding somewhat trivial.