misc: Make AB8500_PWM driver depend on U8500 due to PWM breakage

Since we don't have a PWM API every PWM driver ends up exporting its
own version and we need to limit the platforms we try to build them on
in order to avoid multiple definitions. As the AB8500 is normally a
companion chip for the U8500 CPU depend on that architecture.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>