fit: use 'const' for the input of fdt_offset() and locate_dtb_in_fit()

Those 2 functions don't modify their input, we can mark it const.
This prevents compilation warnings when they are provided const input.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>