Use unicam-legacy driver for Raspberry Pi cameras

All of the Raspberry Pi camera programs and the device tree files
reference the unicam-legacy device driver now. This fixes an issue that
causes cameras to not be found. This regression happened in the Linux
6.12 update due to the unicam->unicam-legacy configuration change
upstream.

Here's an example of it working with this change:

```elixir
iex(1)> cmd("libcamera-jpeg --list")
Available cameras
-----------------
0 : ov5647 [2592x1944 10-bit GBRG] (/base/soc/i2c0mux/i2c@1/ov5647@36)
    Modes: 'SGBRG10_CSI2P' : 640x480 [58.92 fps - (16, 0)/2560x1920 crop]
                                1296x972 [46.34 fps - (0, 0)/2592x1944 crop]
                                1920x1080 [32.81 fps - (348, 434)/1928x1080 crop]
                                2592x1944 [15.63 fps - (0, 0)/2592x1944 crop]

```

Author: fhunleth

linux-6.12.defconfig

@@ -165,7 +165,7 @@ CONFIG_MEDIA_SUPPORT_FILTER=y
 CONFIG_MEDIA_CAMERA_SUPPORT=y
 CONFIG_MEDIA_PLATFORM_SUPPORT=y
 CONFIG_V4L_PLATFORM_DRIVERS=y
-CONFIG_VIDEO_BCM2835_UNICAM=m
+CONFIG_VIDEO_BCM2835_UNICAM_LEGACY=m
 CONFIG_VIDEO_RASPBERRYPI_PISP_BE=m
 CONFIG_VIDEO_IMX219=m
 CONFIG_VIDEO_IMX296=m