arm64: Add HWCAP for self-synchronising virtual counter

Since userspace can make use of the CNTVSS_EL0 instruction, expose
it via a HWCAP.

Suggested-by: Will Deacon <will@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211017124225.3018098-18-maz@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>