mfd: Delete ab3550 driver

The AB3550 never passed the prototype stage. Instead it was used
as a precursor to AB5500 for testing basic building blocks used
in that chip, since they had large similarities. Since AB3550 will
not see the light of day in product form and since the prototypes
are no longer used, let's delete the driver and any references to
it.

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