Merge tag 'u-boot-stm32-20190712' of https://gitlab.denx.de/u-boot/custodians/u-boot-stm

- syscon: add support for power off
- stm32mp1: add op-tee config
- stm32mp1: add specific commands: stboard and stm32key
- add stm32 mailbox driver
- solve many stm32 warnings when building with W=1
- update stm32 gpio driver