board: ti: common: Introduce a common fdt ops library

Introduce a common fdt operations library for basic device tree
operations that are common between various boards.

The first library to introduce here is the capability to set up
fdtfile as a standard variable as part of board identification rather
than depend on scripted ifdeffery.

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Jonathan Humphreys <j-humphreys@ti.com>