ARM64: Enable CONFIG_GENERIC_IRQ_SHOW_LEVEL

Since several interrupt controllers including GIC support both edge and
level triggered interrupts, it's useful to provide that information in
/proc/interrupts even on ARM64 similar to ARM and PPC.

This is based on Geert Uytterhoeven's commit 7c07005e ("ARM: 8339/1:
Enable CONFIG_GENERIC_IRQ_SHOW_LEVEL")

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>