dt-bindings: pm8941-misc: Fix usb_id and usb_vbus definitions

dts validation is throwing an error for me on 8916 and 8939 with
extcon@1300. In this case we have usb_vbus but not usb_id.

Looking at the pm8941-misc driver we can have usb_id, usb_vbus or both at
the same time.

Expand the definition with anyOf to capture the three different valid
modes.

Fixes: 4fcdd677 ("bindings: pm8941-misc: Add support for VBUS detection")
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>