imx_common: check for bmode Serial Downloader

Before commit 81c4eccb ("imx: mx6: fix USB bmode to use
reserved value") a non-reserved value has been used to trigger
Serial Downloader using bmode, which translated to a GPR9 value
of 0x10. However, on some boards the non-reserved value lead to
unreliable bmode command. With the above mentioned commit, U-boot
switched to use [7:4] b0001, which translates to GPR9 0x10 for
Serial Downloader mode. Check for the new value and classify it
as Serial Downloader mode.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
CC: Stefano Babic <sbabic@denx.de>
CC: Tim Harvey <tharvey@gateworks.com>
CC: Fabio Estevam <Fabio.Estevam@freescale.com>
CC: Eric Nelson <eric.nelson@boundarydevices.com>