mips: dts: bcm63268: add TWD block timer

BCM63268 TWD contains block with 3 timers. Add binding for it.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>