board: stm32mp1: add support of nor1 device in dfu command

Add support of mtd backend for nor1 when this device is present on the
board, on STM32MP157C-EV1 for example, as the support of several MTD
spi-nor instance are now supported with commit b7f06056 ("mtd:
spi-nor: allow registering multiple MTDs when DM is enabled").

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>