ARM: dts: am335x-boneblack: add eMMC DT entry

The pinmux is specified in am335x-bone-common.dtsi to be
reused by the eMMC cape.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Tested-by: Kevin Hilman <khilman@linaro.org>
Reviewed-by: Nishanth Menon <nm@ti.com>
[bcousson@baylibre.com: Fix traling spaces and useless comments]
Signed-off-by: Benoit Cousson <bcousson@baylibre.com>