arch: aarch64: Remove useless ISB

Quoting from the reference manual "Writes to PSTATE.{PAN, D, A, I, F}
occur in program order without the need for additional synchronization."
so we can remove the ISB for arch_irq_lock() and arch_irq_unlock().

Signed-off-by: Carlo Caione <ccaione@baylibre.com>