KVM: arm64: Fix missing RES1 in emulation of DBGBIDR

The AArch32 CP14 DBGDIDR has bit 15 set to RES1, which our current
emulation doesn't set. Just add the missing bit.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>