arm64: dts: broadcom: bcm4908: Move reboot syscon out of bus

This fixes following error for every bcm4908 DTS file:
bus@ff800000: reboot: {'type': 'object'} is not allowed for {'compatible': ['syscon-reboot'], 'regmap': [[15]], 'offset': [[52]], 'mask': [[1]]}

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