MIPS: BCM47XX: Fix detected clock on Asus WL520GC and WL520GU

The Asus WL520GC and WL520GU are based on the BCM5354 and clocked at
200MHz, but they do not have a clkfreq nvram variable set to the
correct value. This adds a workaround for these devices.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5843/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>