TF-A Tests: Enable PAuth on warm boot path

This patch provides the following features and makes
modifications listed below:
- `plat_init_apiakey()` function is replaced with `init_apkey()`
  which returns 128-bit value and uses Generic timer physical counter
  value to increase the randomness of the generated key.
  The new function can be used for generation of all ARMv8.3-PAuth keys.
- Source file `pauth.c` moved from `plat/common/aarch64`
  to `lib/extensions/pauth/aarch64` folder which contains PAuth specific
  code.
- Individual APIAKey key generation for each CPU on every warm boot.
- Per-CPU storage of APIAKey added in `tftf_suspend_context` structure.
- APIAKey key is saved/restored in arch context on entry/exit from
  suspended state.
- Added `pauth_init_enable()` function which generates, programs
  and enables APIAKey in EL1/EL2.
- Changes in documentation related to ARMv8.3-PAuth support.

Signed-off-by: Alexei Fedorov <Alexei.Fedorov@arm.com>
Change-Id: I964b8f964bb541cbb0b2f772cb0b07aed055fe36
13 files changed