mfd: ab8500: Rename ab8500 to abx500 for hwmon driver

We are using a generic abx500 hwmon layer, so rename specific ab8500 to generic
abx500 for hwmon device and driver matching.

Signed-off-by: Hongbo Zhang <hongbo.zhang@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>