config: stm32mp15: remove CONFIG_FASTBOOT_USB_DEV

The CONFIG_FASTBOOT_USB_DEV is used to select USB OTG controller other
than 0 but it is not the case for STM32MP15 boards; it can be removed
to simplify the STM32MP15 defconfig files.

On STM32MP15x boards, we have only one USB device with instance 0,
so the device is hardcoded arch/arm/mach-stm32mp/cpu.c with
the command "fastboot 0" and this define is not used in config files
(include/configs/stm32mp15_st_common.h).

Fixes: 4633fd51 ("stm32mp1: activate FASTBOOT on eMMC")
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>