driver: pintcrl: xmc4xxx: Revert recent changes from i2c driver

In commit 541482ff the pinctrl alternate
function mask was increased to also include open-drain setting.

Revert this change because open-drain can already be set via property
drive-open-drain.

The commit also added separate pinctrl nodes for the i2c controller and
target modes. However, the alternate function settings
is the same in both modes, so keep only one and remove the mode
label.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>