libcamera: v4l2_subdevice: Create device from entity name

Add a static method to V4L2Subdevice class to create a V4L2Subdevice
instance from a media entity name.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>

src/libcamera/include/v4l2_subdevice.h

@@ -18,6 +18,8 @@
 
 namespace libcamera {
 
+class MediaDevice;
+
 struct V4L2SubdeviceFormat {
 	uint32_t mbus_code;
 	uint32_t width;
@@ -48,6 +50,9 @@ public:
 	int getFormat(unsigned int pad, V4L2SubdeviceFormat *format);
 	int setFormat(unsigned int pad, V4L2SubdeviceFormat *format);
 
+	static V4L2Subdevice *fromEntityName(const MediaDevice *media,
+					     const std::string &entity);
+
 protected:
 	std::string logPrefix() const;