mfd: Remove ARCH_U8500 dependency from AB8500

While it is vanishingly unlikely that the device will be deployed on other
architectures removing the dependency facilitates build testing when doing
generic work on both the MFD core for the device and the subsystem drivers.

There appears to be no actual code dependency.

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