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 <arnd@arndb.de>
Acked-by: Dave Jones <davej@redhat.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>