arm: stm32mp: add support of STM32MP13x

Introduce the code in mach-stm32mp and the configuration file
stm32mp13_defconfig for the new STM32MP family.

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/MAINTAINERS b/board/st/stm32mp1/MAINTAINERS
index d57a498..c9252e9 100644
--- a/board/st/stm32mp1/MAINTAINERS
+++ b/board/st/stm32mp1/MAINTAINERS
@@ -9,5 +9,7 @@
 F:	configs/stm32mp15_defconfig
 F:	configs/stm32mp15_basic_defconfig
 F:	configs/stm32mp15_trusted_defconfig
+F:	include/configs/stm32mp13_common.h
+F:	include/configs/stm32mp13_st_common.h
 F:	include/configs/stm32mp15_common.h
 F:	include/configs/stm32mp15_st_common.h