This commit imports, squashes and fixes up the following commits by ST:

110dad1 stm32mp1: add USB programmer
d6c7f1f plat/st: initialize dev_status with correct status
78ddbb9 io: io_programmer_st_usb: improve timeout management in usb loop

It excludes the code for the UART programmer.
I used it this way:

make CROSS_COMPILE=arm-none-eabi- PLAT=stm32mp1 ARCH=aarch32 ARM_ARCH_MAJOR=7 AARCH32_SP=sp_min STM32MP_SDMMC=1 STM32MP_EMMC=1 STM32MP_USB_PROGRAMMER=1 DTB_FILE_NAME=stm32mp157c-dk2.dtb DEBUG=1
dfu-util -d 0483:df11 -a 1 -D build/stm32mp1/debug/tf-a-stm32mp157c-dk2.stm32
dfu-util -d 0483:df11 -a 0 -D build/stm32mp1/debug/tf-a-stm32mp157c-dk2.stm32
dfu-util -d 0483:df11 -a 3 -D ~/barebox/images/barebox-stm32mp157c-seeed-odyssey.img
dfu-util -d 0483:df11 -e -a 0