MIPS: BCM47XX: Support Luxul XWR-1750 board

Signed-off-by: Dan Haab <dhaab@luxul.com>
Acked-by: Rafał Miłecki <zajec5@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: Dan Haab <dhaab@luxul.com>
Patchwork: https://patchwork.linux-mips.org/patch/9831/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>