ARM: dts: am335x-evm: disable mmc3

SDIO is not supported in u-boot, there is no point in enabling mmc3.
For this purpose, add u-boot specific dtsi that this will be included
automatically while building the dtb.
Signed-off-by: NJean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: NTom Rini <trini@konsulko.com>

arch/arm/dts/am335x-evm-u-boot.dtsi

+/*
+ * Copyright (C) 2017 Texas Instruments Incorporated - http://www.ti.com/
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+
+&mmc3 {
+	status = "disabled";
+};