KVM: arm64: move ARM-specific defines to uapi/asm/kvm.h

While this in principle breaks userspace code that mentions KVM_ARM_DEV_*
on architectures other than aarch64, this seems unlikely to be
a problem considering that run->s.regs.device_irq_level is only
defined on that architecture.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>