arm64: dts: broadcom: bcm4908: add watchdog block

BCM4908 has the same watchdog as BCM63xx devices. Use "brcm,bcm6345-wdt"
binding which matches the first SoC with that block.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>