west.yml: Update mcumgr revision

Commits affecting Zephyr that are included with the new revision:
  fb489e1f mgmt: Add support for un-registering a group along with
           the same support in img_mgmt
  c8151d80 zephyr: Rely on img_mgmt_find_best_area_id to select update
           partition
  b92aa0b8 Fix encoding usage for halffloat

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>