dt-bindings: extcon: ptn5150: Make 'vbus-gpios' optional

The PTN5150 chip can be used in hardware designs with only reporting of
USB Type-C connection, without the VBUS control.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Vijai Kumar K <vijaikumar.kanagarajan@gmail.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>