ARM: gic: consolidate PPI handling

PPI handling is a bit of an odd beast. It uses its own low level
handling code and is hardwired to the local timers (hence lacking
a registration interface).

Instead, switch the low handling to the normal SPI handling code.
PPIs are handled by the handle_percpu_devid_irq flow.

This also allows the removal of some duplicated code.

Cc: Kukjin Kim <kgene.kim@samsung.com>
Cc: David Brown <davidb@codeaurora.org>
Cc: Bryan Huntsman <bryanh@codeaurora.org>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Magnus Damm <magnus.damm@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Acked-by: David Brown <davidb@codeaurora.org>
Tested-by: David Brown <davidb@codeaurora.org>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>