overlays: Add disable-emmc2

Add a new overlay that disables the EMMC2 controller on BCM2711.
This can be useful on a Compute Module 4 if the onboard EMMC2
storage is unreliable and the system can be booted by other
means e.g Network / USB.

Signed-off-by: Tim Gover <tim.gover@raspberrypi.com>