ixp4xx: timer and clocks cleanups

This patch does a few simple cleanups of the ixp4xx timer
and clocksource/clockevent code in mach-ixp4xx/common.c:

- ixp4xx_clocksource_init() is static and always returns 0,
  which is ignored by its only caller: make it return void
- ixp4xx_clockevent_init(): ditto
- ixp4xx_get_cycles() is only referenced locally: make it static
- use the ixp4xx_timer_irq.dev_id field to pass &clockevent_ixp4xx
  to ixp4xx_timer_interrupt() via its dev_id parameter, allowing
  the code in ixp4xx_timer_interrupt() to be smaller and faster

Tested on an ixp420 machine (ds101).

Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Krzysztof Hałasa <khc@pm.waw.pl>