Merge tag 'u-boot-stm32-20190523' of https://github.com/pchotard/u-boot

Merge tag 'u-boot-stm32-20190523' of https://github.com/pchotard/u-boot

- Add various STM32MP1 fixes for serial, env, clk, board, i2c ...

- Add STM32MP1 DDR driver update:
	These update introduce the DDR interactive mode described in:
	https://wiki.st.com/stm32mpu/index.php/U-Boot_SPL:_DDR_interactive_mode

	This mode is used by the CubeMX: DDR tuning tool.
	https://wiki.st.com/stm32mpu/index.php/STM32CubeMX

	The DDR interactive mode is NOT activated by default because
	it increase the SPL size and slow down the boot time
	(200ms wait added).