SPE is part of the v8.2 architecture, so move its system register and
field definitions into sysreg.h and the new PSB barrier into barrier.h

Finally, move KVM over to using the generic definitions so that it
doesn't have to open-code its own versions.
Acked-by: NMarc Zyngier <marc.zyngier@arm.com>
Acked-by: NMark Rutland <mark.rutland@arm.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>