clk: pxa: export core clocks

pxaxxx_get_clk_frequency_khz() needs several clocks to be available
through clk_get(), ie. the cpu clocks, system bus clock and memory
clocks.

Add the missing clkdev so that their rate can be acquired.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>