iio: accel: adxl345: Extract adxl345_powerup() helper

For the sake of symmetry and possible reuse in the future
extract adxl435_powerup() helper.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Link: https://lore.kernel.org/r/20220222090009.2060-6-andriy.shevchenko@linux.intel.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>