mach-ux500: move CPUfreq driver to cpufreq subsystem

As part of the ARM arch subsystem migration, move the DB8500
cpufreq driver to drivers/cpufreq as discussed with Dave Jones. The
Makefile is not updated in order to avoid cross-subsystem conflicts
for this file in merges.

Cc: Arnd Bergmann <[e-mail]>
Acked-by: Dave Jones <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>