ARM: 8250/1: sa1100: provide OSTIMER0 clock for pxa_timer

Pxa_timer clocksource requires OSTIMER0 clock to be provided.
Add dummy clock returning proper rate.

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