xen/arm: do not read MVFR2 when is not defined

xen/arm: do not read MVFR2 when is not defined

MVFR2 is not available on ARMv7. It is available on ARMv8 aarch32 and
aarch64. If Xen reads MVFR2 on ARMv7 it could crash.

Avoid the issue by doing the following:

- define MVFR2_MAYBE_UNDEFINED on arm32
- if MVFR2_MAYBE_UNDEFINED, do not attempt to read MVFR2 in Xen
- keep the 3rd register_t in struct cpuinfo_arm.mvfr on arm32 so that a
  guest read to the register returns '0' instead of crashing the guest.

'0' is an appropriate value to return to the guest because it is defined
as "no support for miscellaneous features".

Aarch64 Xen is not affected by this patch.

Fixes: 9cfdb489af81 ("xen/arm: Add ID registers and complete cpuinfo")
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Acked-by: Julien Grall <jgrall@amazon.com>