arm64: Add override for ID_AA64MMFR4_EL1.NV_frac

In a very unsurprising turn of events, there is a large class of
firmware that is totally unable to deal with FEAT_NV3, and doesn't
set the required SCR2_EL3.NV3En bit, leading to an UNDEF exception
or an unhandled trap to EL3, depending on the implementation.

Allow the unfortunate user to override ID_AA64MMFR4_EL1.NV_frac
and get a working system. Hopefully firmware will be fixed before
actually HW ships, but I have been there before... :-/

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://patch.msgid.link/20260730071022.296811-30-maz@kernel.org
Signed-off-by: Oliver Upton <oupton@kernel.org>