mvebu: enable /proc/devicetree usage

This is going to be used for proper user-space board identification (ala
mcs814x).

Signed-off-by: Florian Fainelli <florian@openwrt.org>

SVN-Revision: 35207

target/linux/mvebu/config-default

@@ -232,6 +232,7 @@ CONFIG_PLAT_VERSATILE=y
 CONFIG_PLAT_VERSATILE_CLCD=y
 CONFIG_PLAT_VERSATILE_SCHED_CLOCK=y
 # CONFIG_PREEMPT_RCU is not set
+CONFIG_PROC_DEVICETREE=y
 CONFIG_RFS_ACCEL=y
 CONFIG_RPS=y
 CONFIG_RTC_CLASS=y