ARM: dts: am3517: Fix pinmuxing for CD on MMC1

The MMC1 is active low, not active high.  For some reason,
this worked with different combination of U-Boot and kernels,
but it's supposed to be active low and is currently broken.

Fixes: cfaa856a

 ("ARM: dts: am3517: Add pinmuxing, CD and
WP for MMC1") #kernel 4.18+

Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>