BCM270X_DT: Add brcm,bcm2835-sdhci as a fallback

Although downstream uses a different MMC/SDHCI driver there is
no reason why can't support the upstream as a fallback.

See: raspberrypi/firmware#943

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

arch/arm/boot/dts/bcm270x.dtsi

@@ -61,7 +61,7 @@
 		/delete-node/ sdhci@7e300000;
 
 		mmc: mmc@7e300000 {
-			compatible = "brcm,bcm2835-mmc";
+			compatible = "brcm,bcm2835-mmc", "brcm,bcm2835-sdhci";
 			reg = <0x7e300000 0x100>;
 			interrupts = <2 30>;
 			clocks = <&clocks BCM2835_CLOCK_EMMC>;