arm64: Rework setup_cpu_features()

Currently setup_cpu_features() handles a mixture of one-time kernel
feature setup (e.g. cpucaps) and one-time user feature setup (e.g. ELF
hwcaps). Subsequent patches will rework other one-time setup and expand
the logic currently in setup_cpu_features(), and in preparation for this
it would be helpful to split the kernel and user setup into separate
functions.

This patch splits setup_user_features() out of setup_cpu_features(),
with a few additional cleanups of note:

* setup_cpu_features() is renamed to setup_system_features() to make it
  clear that it handles system-wide feature setup rather than cpu-local
  feature setup.

* setup_system_capabilities() is folded into setup_system_features().

* Presence of TTBR0 pan is logged immediately after
  update_cpu_capabilities(), so that this is guaranteed to appear
  alongside all the other detected system cpucaps.

* The 'cwg' variable is removed as its value is only consumed once and
  it's simpler to use cache_type_cwg() directly without assigning its
  return value to a variable.

* The call to setup_user_features() is moved after alternatives are
  patched, which will allow user feature setup code to depend on
  alternative branches and allow for simplifications in subsequent
  patches.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>