arm64: dts: broadcom: bcm4908: Fix timer node for BCM4906 SoC

The cpu mask value in interrupt property inherits from bcm4908.dtsi
which sets to four cpus. Correct the value to two cpus for dual core
BCM4906 SoC.

Fixes: c8b404fb ("arm64: dts: broadcom: bcm4908: add BCM4906 Netgear R8000P DTS files")
Signed-off-by: William Zhang <william.zhang@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>