ARM: stm32: Increase USB power-good delay on DHSOM

The USB hub on STM32MP1 DHCOM boards needs to wait a bit longer until
the USB Vbus is stable. Increase the USB power-good delay to 1 s.

This adds default-undefined STM32MP_BOARD_EXTRA_ENV variable into
stm32mp15_common.h to reduce duplication in board-specific config
files adding custom environment.

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>
diff --git a/include/configs/stm32mp15_common.h b/include/configs/stm32mp15_common.h
index 4e2cabf..dab679f 100644
--- a/include/configs/stm32mp15_common.h
+++ b/include/configs/stm32mp15_common.h
@@ -142,6 +142,10 @@
 	"env_check=if env info -p -d -q; then env save; fi\0" \
 	"boot_net_usb_start=true\0"
 
+#ifndef STM32MP_BOARD_EXTRA_ENV
+#define STM32MP_BOARD_EXTRA_ENV
+#endif
+
 #include <config_distro_bootcmd.h>
 
 /*
@@ -169,7 +173,8 @@
 	STM32MP_BOOTCMD \
 	STM32MP_PARTS_DEFAULT \
 	BOOTENV \
-	STM32MP_EXTRA
+	STM32MP_EXTRA \
+	STM32MP_BOARD_EXTRA_ENV
 
 #endif /* ifndef CONFIG_SPL_BUILD */
 #endif /* ifdef CONFIG_DISTRO_DEFAULTS*/