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

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 <[email protected]>
Reviewed-by: Tom Rini <[email protected]>
Reviewed-by: Simon Glass <[email protected]>