ARM: 8251/1: clocksource: enable pxa_timer for SA-1100 platform

SA-11x0 platform used the same IP block as was used on PXA. Consequently
it makes sense to have only one driver. Enable pxa_timer clocksource for
StrongARM platform.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>