fastboot: Add CONFIG_FASTBOOT_USB_DEV option

Some boards (like AM57x EVM) has USB OTG controller other than 0. So in
order to use correct controller number in compiled environment we should
define CONFIG_FASTBOOT_USB_DEV option.

For example, when doing "fastboot reboot-bootloader" we want to enter
fastboot mode automatically. But to do so we need to provide controller
number to "fastboot" command. If this procedure is defined in some config
which is common to bunch of boards, and boards have different USB
controller numbers, we can't just hardcode "fastboot 0" in the
environment. We need to use configurable option, which this patch adds.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>