mfd: Remove db8500-prcmu U8400 legacy

This removes the U8400 legacy from PRCMU and cpufreq drivers.
This platform has no current in-kernel users.

Signed-off-by: Daniel Willerud <daniel.willerud@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>