`mw 0x20d8040 0x00000010; mw 0x20d8044 0x10000000; reset` issued on an
i.MX6Q forces serial download mode, but there is no indication of that
in the sbmr2 register, so barebox reports $bootsource=unknown.

Similarly, `mw 0x20d8040 0x00000020; mw 0x20d8044 0x10000000; reset`
forces an i.MX6UL into recovery mode after reset.

Do as U-Boot does and interpret the (reserved) value in BOOT_CFG1(7, 4)
for each SoC as serial download.

Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>