iwlwifi: more modularity in fw images and sections

Changed iwl_firmware_pieces structure to support an array of
separate images, and an array of sections for each image.

In fw_sec and fw_desc structures, added a field for
offset from the HW address, to support 16.0 uCode that
provides an offset instead of any other data about the section.
This field is filled with default values when parsing instruction
or data section.

Signed-off-by: David Spinadel <david.spinadel@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>