cbfs: Unify the two header loaders

These two functions have mostly the same code. Pull this out into a common
function.

Also make this function zero the private data so that callers don't have
to do it. Finally, update cbfs_load_header_ptr() to take the base of the
ROM as its parameter, which makes more sense than passing the address of
the header within the ROM.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>