ARM: stm32: Align mtdparts with SPI NOR layout on DHSOM

The DHSOM uses different SPI NOR layout than the ST devkit, stop
pulling in the ST specific runtime mtdparts settings and adjust
the mtdparts accordingly.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>