Merge tag 'u-boot-stm32-20200214' of https://gitlab.denx.de/u-boot/custodians/u-boot-stm

- add DH Electronics DHCOM SoM and PDK2 board
- DT alignment with kernel v5.5-rc7 for stm32mp1 boards
- fix STM32 image format for big endian hosts in mkimage
- solve warnings in device tree and code for stm32mp1 boards
- remove fdt_high and initrd_high for stm32 and stih boards
- add support of STM32MP15x Rev.Z
- update stm32mp1 readme