MIPS: BCM63XX: Fix platform_devices id

There is only one watchdog and VoIP DSP platform devices per board, use
-1 as the platform_device id accordingly.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3313/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>