docs: arm64: Document EL3 requirements for FEAT_PMUv3

docs: arm64: Document EL3 requirements for FEAT_PMUv3

This documents EL3 requirements for FEAT_PMUv3. The register field MDCR_EL3
.TPM needs to be cleared for accesses into PMU registers without any trap
being generated into EL3. PMUv3 registers like PMCCFILTR_EL0, PMCCNTR_EL0
PMCNTENCLR_EL0, PMCNTENSET_EL0, PMCR_EL0, PMEVCNTR<n>_EL0, PMEVTYPER<n>_EL0
etc are already being accessed for perf HW PMU implementation.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lore.kernel.org/r/20241211065425.1106683-3-anshuman.khandual@arm.com
Signed-off-by: Will Deacon <will@kernel.org>