dt-bindings: media: s5c73m3: Fix reset-gpio descriptor

The reset-gpios is described as xshutdown-gpios on the required
properties, as it is on the driver. Despite that, the device tree
example set the property 'reset-gpios' instead of the property
'xshutdown-gpios'.

Therefore, this patch updates the example to match the property specified
on the driver.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>