ID_AA64MMFR2_EL1 is all zeros
Description of problem:
When the `ID_AA64MMFR2_EL1` register is read via `mrs x[n], ID_AA64MMFR2_EL1`, it is read as all zeros. This is at the very least not correct for `ID_AA64MMFR2_EL1.ST`, which describes support for small translation tables (FEAT_TTST).
Steps to reproduce:
1. Run `mrs x[n], ID_AA64MMFR2_EL1` within qemu-system-aarch64
Additional information:
FEAT_TTST is a relatively new aarch64 feature that appears to have caused many problems basically everywhere. However, [qemu has reportedly implemented it](https://www.qemu.org/2021/04/30/qemu-6-0-0/).