ARM: sa11x0/pxa: get rid of get_clock_tick_rate

The last user of this function is gone, so remove it. The clock API
should now be used to get clock rates.

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>