ar71xx: Fix 2.4G WiFi LED of the OpenMesh MR600 board

The OpenMesh MR600(v1) can only enable the 2.4G WiFi PHY LED through the
mini-PCIe device. Not configuring the LED pin inside the platform data
makes it impossible to configure it through any standard OpenWrt tool.

Signed-off-by: Sven Eckelmann <sven@open-mesh.com>

Backport of r42184

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42363 3c298f89-4303-0410-b956-a3cf2f4a3e73