[ARM] 4550/1: sched_clock on PXA should cope with run time clock rate selection

The previous implementation was relying on compile time optimizations
based on a constant clock rate.  However, support for different PXA
flavors in the same kernel binary requires that the clock be selected at
run time, so here it is.

Let's move this code to a more appropriate location while at it.

Signed-off-by: Nicolas Pitre <npitre@mvista.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>