mfd: intel_soc_pmic_bxtwc: Create sysfs attributes using core driver's facility

Driver core takes care of sysfs attributes. Use this facility instead of
doing it explicitly in ->probe() and ->remove().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20220628221747.33956-2-andriy.shevchenko@linux.intel.com