This is currently called by DCZVA with all zero tag bits and was causing
assertions while running cheribsdtest in Jenkins. The TLBENTRYCAP_FLAG_TRAP
prevents writing non-zero tag bits, but the assertion was triggering for
any value of tags, so relax it to actually model the required condition.

Previously both purecap-kernel and hybrid-kernel jobs were booting the
default kernel (currently hybrid), which is not right for the purecap
jobs. For Morello purecap this did not matter since the kernel is chosen
from the loader prompt, but RISC-V boots the kernel directly.