dt-bindings: Require child nodes type to be 'object'

A node is always an object (aka a dictionary), so make that explicit for
child node schemas.

A meta-schema update will enforce having 'type' specified.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>