board: stmp32mp1: split board and SOC STM32MP15 configuration

Add a configuration file "stm32mp15_st_common.h" to handle the
STMicroelectronics boards configuration and rename stm32mp1.h to
"stm32mp15_common.h" to handle the generic STM32MP15x series configuration.

The configuration file "dh_stm32mp1.h" is also renamed to
"stm32mp15_dh_dhcom" for the configuration of board based on the
dhelectronics STM32MP15x SOM.

In the STMicroelectronics board configuration the default bootcmd
"bootcmd_stm32mp" is updated to only select the extlinux file found on
SD-Card on STM32MP15x EV1 for boot from NOR device.

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