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>
diff --git a/board/st/stm32mp1/Kconfig b/board/st/stm32mp1/Kconfig
index c5ab755..89e97ae 100644
--- a/board/st/stm32mp1/Kconfig
+++ b/board/st/stm32mp1/Kconfig
@@ -7,7 +7,7 @@
 	default "st"
 
 config SYS_CONFIG_NAME
-	default "stm32mp1"
+	default "stm32mp15_st_common"
 
 source "board/st/common/Kconfig"
 endif
diff --git a/board/st/stm32mp1/MAINTAINERS b/board/st/stm32mp1/MAINTAINERS
index 0e6d80f..6451195 100644
--- a/board/st/stm32mp1/MAINTAINERS
+++ b/board/st/stm32mp1/MAINTAINERS
@@ -8,4 +8,5 @@
 F:	configs/stm32mp15_defconfig
 F:	configs/stm32mp15_basic_defconfig
 F:	configs/stm32mp15_trusted_defconfig
-F:	include/configs/stm32mp1.h
+F:	include/configs/stm32mp15_common.h
+F:	include/configs/stm32mp15_st_common.h