KVM: arm64: Use symbolic definition for ISR_EL1.A

Now that we are generating ISR_EL1 we have acquired a constant for
ISR_EL1.A, use it rather than the magic number we had been using in the KVM
entry code.

Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>