ARM: sa11x0/pxa: acquire timer rate from the clock rate

As both pxa and sa1100 provide a clock to the timer, the rate can be
inferred from the clock rather than hard encoded in a functional call.

This patch changes the pxa timer to have a mandatory clock which is used
as the timer rate.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Russell King <rmk+kernel@armlinux.org.uk>