The i.MX6UL differs from the i.MX6Q in the interpretation of the first
two values for BOOT_CFG(7, 4):

+--------------+----------+----------+
|BOOT_CFG1(7,4)|   0x01   |   0x02   |
|==============+==========+==========|
| MX6Q         | reserved |   SATA   |
|--------------+----------+----------|
| MX6UL        |   QSPI   | reserved |
+--------------+----------+----------+

The reserved (forced serial) values are handled in the previous commit.
Add QSPI boot source detection now.

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