MIPS: BCM47XX: Add detection and GPIO config for Siemens SE505v2

This adds board detection for the Siemens SE505v2 and the led gpio
configuration. This board does not have any buttons.
This is based on OpenWrt broadcom-diag and Manuel Munz's nvram dump.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Cc: linux-mips@linux-mips.org
Cc: zajec5@gmail.com
Patchwork: https://patchwork.linux-mips.org/patch/6593/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>