clk: add pxa25x clock drivers

Move pxa25x clock drivers from arch/arm/mach-pxa to driver/clk.
In the move :
 - convert to new clock framework legacy clocks
 - provide clocks as before for platform data based boards
 - provide clocks through devicetree with clk-pxa-dt

This is the preliminary step in the conversion. The remaining steps are
:
 - pxa3xx
 - once PXA is fully converted to device tree, if that happens,
   clk-pxa2* and clk-pxa3* should only hold the core clocks which cannot
   be described in devicetree.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Tested-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Michael Turquette <mturquette@linaro.org>