Despite both being documented as read-only registers, only one of them
is truly read-only. An mtspr to HID1 will steamroll bits 0-4 with
bits 0-4 of whatever value is currently in the source register, the rest
of the bits are not modified as bits 5-31 are considered reserved, so
these ignore writes to them.
PVR on the other hand, is truly a read-only register. Attempts to write
to it don't modify the value within it, so we model this behavior.
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