overlays: Add sdtweak features for network booting

It has been observed that a Pi with no SD card will poll the interface
continuously, using up to 10% CPU. Add some new parameters to the
sdtweak overlay to control this behaviour:

poll_once   Only look for a card once, at boot time. If none is found
            then the interface is effectively disabled.

enable      Set to "off" or "no" to completely disable the interface.

See: #2567

Signed-off-by: Phil Elwell <phil@raspberrypi.org>

arch/arm/boot/dts/overlays/README

@@ -1618,6 +1618,16 @@ Params: overclock_50            Clock (in MHz) to use when the MMC framework
 
         debug                   Enable debug output (default off)
 
+        poll_once               Looks for a card once after booting. Useful
+                                for network booting scenarios to avoid the
+                                overhead of continuous polling. N.B. Using
+                                this option restricts the system to using a
+                                single card per boot (or none at all).
+                                (default off)
+
+        enable                  Set to off to completely disable the interface
+                                (default on)
+
 
 Name:   smi
 Info:   Enables the Secondary Memory Interface peripheral. Uses GPIOs 2-25!

arch/arm/boot/dts/overlays/sdtweak-overlay.dts

@@ -19,5 +19,7 @@
 		force_pio        = <&frag0>,"brcm,force-pio?";
 		pio_limit        = <&frag0>,"brcm,pio-limit:0";
 		debug            = <&frag0>,"brcm,debug?";
+		enable           = <&frag0>,"status";
+		poll_once        = <&frag0>,"non-removable?";
 	};
 };