ARM: pxa: auto compute shift and mult of timer

Use auto-computed shift and mult of timer with new API
clocksource_calc_mult_shift()/clockevents_calc_mult_shift().

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Cc: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>