arm64/sve: Detect SVE and activate runtime support

This patch enables detection of hardware SVE support via the
cpufeatures framework, and reports its presence to the kernel and
userspace via the new ARM64_SVE cpucap and HWCAP_SVE hwcap
respectively.

Userspace can also detect SVE using ID_AA64PFR0_EL1, using the
cpufeatures MRS emulation.

When running on hardware that supports SVE, this enables runtime
kernel support for SVE, and allows user tasks to execute SVE
instructions and make of the of the SVE-specific user/kernel
interface extensions implemented by this series.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>

---

Dropped Suzuki's Reviewed-by, since there are non-trivial changes.

Changes since v2
----------------

Requested by Catalin Marinas:

 * Moved the EL1 SVE trap disable logic to cpufeatures.
   This requires the trap to be disabled when the ARM64_SVE cpu
   capability is detected on any booting cpu.