Decode the arm64 SVE ID register

The field values are only valid when the ID_AA64PFR0_EL1.SVE or
ID_AA64PFR1_EL1.SME vields are non-zero. When this is not the case
the register is reserved as zero so is safe to read, but the SVEver
field will be incorrect so only print the decoded register when
the SVE or SME fields indicate it is valid.

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