arm64: dts: broadcom: bcm4908: Fix cpu node for smp boot

Add spin-table enable-method and cpu-release-addr properties for
cpu0 node. This is required by all ARMv8 SoC. Otherwise some
bootloader like u-boot can not update cpu-release-addr and linux
fails to start up secondary cpus.

Fixes: 2961f69f ("arm64: dts: broadcom: add BCM4908 and Asus GT-AC5300 early DTS files")
Signed-off-by: William Zhang <william.zhang@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>