KVM: arm: Reserve bit in KVM_REG_ARM encoding for secure/nonsecure

We have a KVM_REG_ARM encoding that we use to expose KVM guest registers
to userspace. Define that bit 28 in this encoding indicates secure vs
nonsecure, so we can distinguish the secure and nonsecure banked versions
of a banked AArch32 register.

For KVM currently, all guest registers are nonsecure, but defining
the bit is useful for userspace. In particular, QEMU uses this
encoding as part of its on-the-wire migration format, and needs to be
able to describe secure-bank registers when it is migrating (fully
emulated) EL3-enabled CPUs.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>