mfd: Renamed ab3100.h to abx500.h

The goal here is to make way for a more general interface for the
analog baseband chips ab3100 ab3550 ab550 and future chips.

This patch have been divided into two parts since both changing name
and content of a file is not recommended in git.

Signed-off-by: Mattias Wallin <mattias.wallin@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>