clk: pxa: keep clocks initialization separated per variant

Have each pxa variant (pxa25x, pxa27x, pxa3xx) have its own device-tree
clock initializing function, to be able to register its own specific
core clocks.

Apply that change specifically to pxa27x.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Michael Turquette <mturquette@linaro.org>