ARM: dts: am335x-bone: add CD for mmc1

This enables the use of MMC cards even when no card was inserted at boot.

Signed-off-by: Alexander Holler <holler@ahsoftware.de>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Tested-by: Kevin Hilman <khilman@linaro.org>
Reviewed-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Benoit Cousson <bcousson@baylibre.com>