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>
ARM: am335x-bone-common: switch mmc1 to 4-bit mode

The micro-SD slot hooks up all four data pins so lets' use them.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
ARM: dts: am335x-bone-common: add cpu0 and mmc1 triggers

This matches the vendor 3.8.x configuration that is shipping with the boards.

The LED layout is now:

USR0: heartbeat
USR1: mmc0 (micro-SD slot)
USR2: cpu0
USR3: mmc1 (eMMC)

The cpu0 triggers was put inbetween the mmc triggers to make is easier to see
where the disk activity is.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>