dt-bindings: Ensure child nodes are of type 'object'

Properties which are child node definitions need to have an explict
type. Otherwise, a matching (DT) property can silently match when an
error is desired. Fix this up tree-wide. Once this is fixed, the
meta-schema will enforce this on any child node definitions.

Cc: Chen-Yu Tsai <wens@csie.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Brian Norris <computersforpeace@gmail.com>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: linux-mtd@lists.infradead.org
Cc: linux-gpio@vger.kernel.org
Cc: linux-stm32@st-md-mailman.stormreply.com
Cc: linux-spi@vger.kernel.org
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Acked-by: Mark Brown <broonie@kernel.org>
Acked-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: Rob Herring <robh@kernel.org>