arm: Break cyclic percpu include

In order to use <asm/percpu.h> in irqflags.h, we need to make sure
asm/percpu.h does not itself depend on irqflags.h.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lkml.kernel.org/r/20200623083721.454517573@infradead.org