arm64/fpsimd: Only provide the length to cpufeature for xCR registers

arm64/fpsimd: Only provide the length to cpufeature for xCR registers

For both SVE and SME we abuse the generic register field comparison
support in the cpufeature code as part of our detection of unsupported
variations in the vector lengths available to PEs, reporting the maximum
vector lengths via ZCR_EL1.LEN and SMCR_EL1.LEN.  Since these are
configuration registers rather than identification registers the
assumptions the cpufeature code makes about how unknown bitfields behave
are invalid, leading to warnings when SME features like FA64 are enabled
and we hotplug a CPU:

  CPU features: SANITY CHECK: Unexpected variation in SYS_SMCR_EL1. Boot CPU: 0x0000000000000f, CPU3: 0x0000008000000f
  CPU features: Unsupported CPU feature variation detected.

SVE has no controls other than the vector length so is not yet impacted
but the same issue will apply there if any are defined.

Since the only field we are interested in having the cpufeature code
handle is the length field and we use a custom read function to obtain
the value we can avoid these warnings by filtering out all other bits
when we return the register value, if we're doing that we don't need to
bother reading the register at all and can simply use the RDVL/RDSVL
value we were filling in instead.

Fixes: 2e0f2478ea37 ("arm64/sve: Probe SVE capabilities and usable vector lengths")
FixeS: b42990d3bf77 ("arm64/sme: Identify supported SME vector lengths at boot")
Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20230731-arm64-sme-fa64-hotplug-v2-1-7714c00dd902@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>