cmd: fdt: Use env_set_hex() for "get addr" and "get size"

The 'fdt get addr' and 'env get size' is always assumed to be hex
value, drop the prefix, and outright switch to env_set_hex(). Since
this might break existing users who depend on the existing behavior
with 0x prefix, this is a separate patch.

Revert if this breaks anything.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>