mfd: Update kconfig for ab8500 core driver

This patch add a dependancy for ab8500-core driver so as to depend on
u8500 platform.

This patch also fixes the build issues(powerpc_allyesconfig) for the
patch 03f582a93ecca6e9584b622570022abf08ed03ec (misc: Add ab8500 pwm
driver)

Signed-off-by: Arun Murthy <arun.murthy@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>