KVM: arm64: pmu: Use generated define for PMSELR_EL0.SEL access

KVM: arm64: pmu: Use generated define for PMSELR_EL0.SEL access

ARMV8_PMU_COUNTER_MASK is really a mask for the PMSELR_EL0.SEL register
field. Make that clear by adding a standard sysreg definition for the
register, and using it instead.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Tested-by: James Clark <james.clark@linaro.org>
Link: https://lore.kernel.org/r/20240731-arm-pmu-3-9-icntr-v3-4-280a8d7ff465@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>