overlays: Add "always-on" parameter to imx477 and imx296

Leave the camera's power supplies up, to prevent the camera
clamping its 1.8V digital I/Os to ground. This may be useful
when synchronizing multiple camera systems using XVS or XTRIG.

Signed-off-by: Nick Hollinghurst <nick.hollinghurst@raspberrypi.com>