usb, g_dnl: make bcdDevice value configurable

add the possibility to set the bcdDevice number board specific.
Therefore the weak function g_dnl_get_board_bcd_device_number()
is introduced. Used on the siemens boards.

Signed-off-by: Heiko Schocher <hs@denx.de>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>