media: atomisp: Refactor PMIC detection to a separate function

Refactor PMIC detection to a separate function. In the future
we may move this code somewhere else where it's more suitable.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>