arm64: dts: broadcom: bcm4908: use proper TWD binding

Block at <ff800400 0x4c> is a TWD that contains timers, watchdog and
reset. Actual timers happen to be at block beginning but they only span
across the first 0x28 registers. It means the old block description was
incorrect (size 0x3c).

Drop timers binding for now and use documented TWD binding. Timers
should be properly documented and defined as TWD subnode.

Fixes: 2961f69f ("arm64: dts: broadcom: add BCM4908 and Asus GT-AC5300 early DTS files")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>