arm64: Add PMUv3 support

In order to enable the in-kernel PMU emulation code, add a tiny bit
of setup code that initializes the PMU on each CPU and populates
the DT. The IRQ is harcoded to PPI7 (INTID23) in order to match
what QEMU does.

The code is enabled when the --pmu option is passed to lkvm.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>