Merge branch '2023-06-09-fwu-updates' into next

Merge branch '2023-06-09-fwu-updates' into next

Two sets of FWU updates from Jassi Brar.  First:
The patchset reduces ~400 lines of code, while keeping the functionality same and making
meta-data operations much faster (by using cached structures).

Issue:
meta-data copies (primary and secondary) are being handled by the
backend/storage layer instead of the common core in fwu.c (as also noted by
Ilias)  that is, gpt_blk.c manages meta-data and similarly raw_mtd.c will have
to do the same when it arrives. The code could by make smaller, cleaner and
optimised.

Basic idea:
Introduce  .read_mdata() and .write_mdata() in fwu_mdata_ops  that simply
read/write meta-data copy. The core code takes care of integrity and redundancy
of the meta-data, as a result we can get rid of every other callback
.get_mdata() .update_mdata() .get_mdata_part_num()  .read_mdata_partition()
.write_mdata_partition() and the corresponding wrapper functions thereby making
the code 100s of LOC smaller.

Get rid of fwu_check_mdata_validity() and fwu_mdata_check() which expected
underlying layer to manage and verify mdata copies.
Implement  fwu_get_verified_mdata(struct fwu_mdata *mdata) public function that
reads, verifies and, if needed, fixes the meta-data copies.

Verified copy of meta-data is now cached as 'g_mdata' in fwu.c, which avoids
multiple low-level expensive read and parse calls.
gpt meta-data partition numbers are now cached in gpt_blk.c, so that we don't
have to do expensive part_get_info() and uid ops.

And second:
Introduce support for mtd backed storage for FWU feature and enable it on
Synquacer platform based DeveloperBox.