Merge changes from topic "bk/warnings" into integration

* changes:
  docs: describe the new warning levels
  build: add -Wunused-const-variable=2 to W=2
  build: include -Wextra in generic builds
  docs(porting-guide): update a reference
  fix(st-usb): replace redundant checks with asserts
  fix(brcm): add braces around bodies of conditionals
  fix(renesas): align incompatible function pointers
  fix(zynqmp): remove redundant api_version check
  fix: remove old-style declarations
  fix: unify fallthrough annotations

diff --cc Makefile
Simple merge

diff --cc docs/getting_started/build-options.rst
Simple merge

diff --cc docs/getting_started/porting-guide.rst
Simple merge

diff --cc plat/xilinx/zynqmp/pm_service/pm_svc_main.c
Simple merge