cpufreq: pxa3: move clk register access to clk driver

The driver needs some low-level register access for setting
the core and bus frequencies. These registers are owned
by the clk driver, so move the low-level access into that
driver with a slightly higher-level interface and avoid
any machine header file dependencies.

Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Cc: linux-clk@vger.kernel.org
Cc: linux-pm@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>