ARM: dts: stm32: Remove extra size-cells on dhcom-pdk2

Fix make dtbs_check warning:
arch/arm/boot/dts/stm32mp157c-dhcom-pdk2.dt.yaml: gpio-keys-polled: '#address-cells' is a dependency of '#size-cells'
arch/arm/boot/dts/stm32mp157c-dhcom-pdk2.dt.yaml: gpio-keys: '#address-cells' is a dependency of '#size-cells'

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: linux-stm32@st-md-mailman.stormreply.com
To: linux-arm-kernel@lists.infradead.org

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>