ARM: dts: stm32: Update GPIO line names on PicoITX

Use more specific custom GPIO line names which denote exactly where
the GPIO came from, i.e. the base board. Also, update the new blank
GPIO line names set up by stm32mp15xx-dhcom-som.dtsi back to their
original values.

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>