mfd: intel_soc_pmic_bxtwc: Switch to use ATTRIBUTE_GROUPS()

Embrace ATTRIBUTE_GROUPS() to avoid boiler plate code.
While at it, move DEVICE_ATTR_ADMIN_RW() closer to the callbacks.

This should not introduce any functional changes.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20241016105201.757024-2-andriy.shevchenko@linux.intel.com


Signed-off-by: Lee Jones <lee@kernel.org>