ARM: pxa: remove get_clk_frequency_khz()

get_clk_frequency_khz() is not a proper name for a global function,
and there is only one caller.

Convert viper to use the properly namespaced
pxa25x_get_clk_frequency_khz() and remove the other references.

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: linux-pm@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>