docs: fpga: mgr: document parse_header() callback

Document newly introduced fpga_manager_ops callback parse_header() and
flag skip_header along with header_size and data_size fields of struct
fpga_image_info.

Signed-off-by: Ivan Bornyakov <i.bornyakov@metrotek.ru>
Acked-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/20220623163248.3672-3-i.bornyakov@metrotek.ru


Signed-off-by: Xu Yilun <yilun.xu@intel.com>