ARM: dts: stm32: Add GPIO keys for STM32MP1 DHCOM PDK2

Add bindings for the four GPIO keys on DH PDK2 board. Note that TA1
key is polled because it's IRQ line conflicts with ethernet IRQ, the
rest of the GPIO keys, TA2, TA3, TA4, are interrupt-driven and wake
up sources.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.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: Alexandre Torgue <alexandre.torgue@st.com>