[MIPS] GPIO LED driver for the WGT634U machine

Add LED support to the WGT634U machine.  It uses the new gpio-led
driver and a platform driver for the pin definitions.
    
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>