lib: utils/regmap: Add simple FDT based regmap framework

We add a simple FDT based regmap framework which is built on top of
generic regmap library. The phandle of FDT regmap DT node is treated
as unique regmap ID. The FDT based regmap drivers will be probed
on-demand from fdt_regmap_get_by_phandle() and fdt_regmap_get()
called by the regmap client drivers.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Xiang W <wxjstz@126.com>