irqchip/gic-v3-its: Fix build for !SMP

Commit 835f442f ("irqchip/gic-v3-its: Limit memreserve cpuhp state
lifetime") added a reference to cpus_booted_once_mask, which does not
exist on !SMP builds, breaking the build for such configurations.

Given the intent of the check, short circuit it to always pass.

Cc: Valentin Schneider <valentin.schneider@arm.com>
Fixes: 835f442f

 ("irqchip/gic-v3-its: Limit memreserve cpuhp state lifetime")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220122151614.133766-1-ardb@kernel.org