target/arm: Implement ARMv8.1-VMID16 extension

The ARMv8.1-VMID16 extension extends the VMID from 8 bits to 16 bits:

 * the ID_AA64MMFR1_EL1.VMIDBits field specifies whether the VMID is
   8 or 16 bits
 * the VMID field in VTTBR_EL2 is extended to 16 bits
 * VTCR_EL2.VS lets the guest specify whether to use the full 16 bits,
   or use the backwards-compatible 8 bits

For QEMU implementing this is trivial:
 * we do not track VMIDs in TLB entries, so we never use the VMID field
 * we treat any write to VTTBR_EL2, not just a change to the VMID field
   bits, as a "possible VMID change" that causes us to throw away TLB
   entries, so that code doesn't need changing
 * we allow the guest to read/write the VTCR_EL2.VS bit already

So all that's missing is the ID register part: report that we support
VMID16 in our 'max' CPU.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200210120146.17631-1-peter.maydell@linaro.org

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