arm64: dts: specify 1.8V EMMC capabilities for bcm958742t

Specify 1.8V EMMC capabilities for bcm958742t board to indicate support
for UHS mode.

Fixes: d4b4aba6 ("arm64: dts: Initial DTS files for Broadcom Stingray SOC")
Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

arch/arm64/boot/dts/broadcom/stingray/bcm958742t.dts

@@ -42,3 +42,7 @@ / {
 &gphy0 {
 	enet-phy-lane-swap;
 };
+
+&sdio0 {
+	mmc-ddr-1_8v;
+};