usb, g_dnl: make iSerialNumber board configurable

add the possibility to set the iSerialNumber board specific.
Default value for iSerialNumber is 0x0. This value can
changed board specific through the new function
g_dnl_set_serialnumber() which must be called from the
board specific function g_dnl_bind_fixup().

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