ARM: DTS: am3517-evm-u-boot: Mark MMC1 with cd-inverted

In order to use the device tree for MMC, the card-detect pin
needs to be inverted.  This patch places this into the
am3517-evm-u-boot.dtsi file to keep the main DTS and DTSI files
clean and in-sync with Linux

Signed-off-by: Adam Ford <aford173@gmail.com>