ARM: irq: Do not clear pending IRQ when enabling them.

Do not clear pending IRQ when enabling an IRQ on ARM
architectures. Peripherals and S/W ISRs may be required to
be deferred until they are enabled later and the pended
IRQ should be retained for the ISR to be vectored to when
enabled.

Change-id: I808183018d8a2cc58390a1de3b4797b2bb7c6ec9
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>