linux-phytec-ti

Input: axp20x-pek - switch over to using attribute group

Instead of registering device attributes individually let's use attribute
groups and also devm_* infrastructure to ease cleanup.

Tested-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>