mfd: Remove default selection of AB4500

This remove the default 'y' selection of AB4500, currently everyone
enabling the SPI subsystem will get a copy of this driver.

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Cc: Srinidhi Kasagar <srinidhi.kasagar@stericsson.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>