arm: vt8500: doc: Add device tree bindings for arch-vt8500 devices

Bindings for gpio, interrupt controller, power management controller,
timer, realtime clock, serial uart, ehci and uhci controllers and
framebuffer controllers used on the arch-vt8500 platform.

Framebuffer binding also specifies a 'display' node which is required
for determining the lcd panel data.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>