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

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 <[email protected]>
Acked-by: Daniel Lezcano <[email protected]>
Acked-by: Russell King <[email protected]>