Merge changes from topic "psci-osi" into integration

* changes:
  feat(sc7280): add support for PSCI_OS_INIT_MODE
  feat(fvp): enable support for PSCI OS-initiated mode
  feat(psci): update PSCI_FEATURES
  feat(psci): add support for OS-initiated mode
  feat(psci): add support for PSCI_SET_SUSPEND_MODE
  build(psci): add build option for OS-initiated mode
  docs(psci): add design proposal for OS-initiated mode

diff --cc Makefile
Simple merge

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

diff --cc make_helpers/defaults.mk
Simple merge

diff --cc plat/arm/board/fvp/fvp_pm.c
Simple merge

diff --cc plat/arm/board/fvp/platform.mk
Simple merge