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

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>