ARM: dts: stm32: Fill GPIO line names on DHCOM SoM

ARM: dts: stm32: Fill GPIO line names on DHCOM SoM

Fill in the custom GPIO line names used by DH on the DHCOM SoM.
The GPIO line names are in accordance to DHCOM Design Guide R04
available at [1], section 3.9 GPIO.

[1] https://wiki.dh-electronics.com/images/5/52/DOC_DHCOM-Design-Guide_R04_2018-06-28.pdf

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