KVM: PPC: Make EPCR a valid field for booke64 and bookehv

In BookE, EPCR is defined and valid when either the HV or the 64bit
category are implemented. Reflect this in the field definition.

Today the only KVM target on 64bit is HV enabled, so there is no
change in actual source code, but this keeps the code closer to the
spec and doesn't build up artificial road blocks for a PR KVM
on 64bit.

Signed-off-by: Alexander Graf <agraf@suse.de>