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 that case we have usb_vbus but not usb_id.

It wasn't immediately obvious if there was a valid use-case for the
existing code for usb_id in isolation, however discussing further, we
concluded that usb_id, usb_vbus or (usb_id | usb_vbus) are valid
combinations as an external IC may be responsible for usb_id or usb_vbus.

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

Fixes: 4fcdd677c4ea ("bindings: pm8941-misc: Add support for VBUS detection")
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>