fdt_support: move fdt_valid from cmd_fdt.c to fdt_support.c

Move the fdt_valid function to fdt_support.
This changes allow to be able to test the validity of a devicetree in
other c files.

Update code syntax.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Maxime Ripard <maxime@cerno.tech>