ARM: mach-virt: Select PMUv3 driver by default

Since 32bit guests are not unlikely to run on an ARMv8 host,
let's select the PMUv3 driver, which allows the PMU to be used
on such systems.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Zaid Al-Bassam <zalbassam@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230126204444.2204061-9-zalbassam@google.com