ARM: dts: Add sd_poll_once dtparam to bcm283x/2711

The old sdtweak overlay allowed the SD interface to be effectively
disabled unless there was a card present at boot time, but that
overlay doesn't work on bcm2711 and has largely been replaced by
a set of sd_* dtparams (which have the advantage of being board-
specific.

Add an sd_poll_once dtparam to allow the same functionality on
all Raspberry Pi boards.

See: #3286

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

arch/arm/boot/dts/bcm2708-rpi.dtsi

@@ -92,6 +92,7 @@
 		watchdog = <&watchdog>,"status";
 		random = <&random>,"status";
 		sd_overclock = <&sdhost>,"brcm,overclock-50:0";
+		sd_poll_once = <&sdhost>,"non-removable?";
 		sd_force_pio = <&sdhost>,"brcm,force-pio?";
 		sd_pio_limit = <&sdhost>,"brcm,pio-limit:0";
 		sd_debug     = <&sdhost>,"brcm,debug";

arch/arm/boot/dts/bcm2711-rpi-4-b.dts

@@ -363,5 +363,7 @@
 
 		eth_led0 = <&phy1>,"led-modes:0";
 		eth_led1 = <&phy1>,"led-modes:4";
+
+		sd_poll_once = <&emmc2>, "non-removable?";
 	};
 };

arch/arm/boot/dts/overlays/README

@@ -165,6 +165,13 @@ Params:
         sd_overclock            Clock (in MHz) to use when the MMC framework
                                 requests 50MHz
 
+        sd_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)
+
         sd_force_pio            Disable DMA support for SD driver (default off)
 
         sd_pio_limit            Number of blocks above which to use DMA for