The i.MX6 series SoC type is determined by barebox by examining the
USB_ANALOG_DIGPROG aka IMX6_ANATOP_SI_REV register. This register is located
at a common offset for all mx6 SoC - except for i.MX6SL where a different
offset is used. This creates a dilemma while distinguishing the mx6sl from
non-mx6sl SOC since the SoC type identification register location is type
specific itself.

Access to undocumented and probably invalid or unpredictable registers should
be avoided as possible. For the mx6sl detection an access to the general
USB_ANALOG_DIGPROG @0x260 cannot be avoided when running on mx6sl. This
register contained the value 0x00014009 for different mx6sl Rev. 1.2 based
e-book readers using MCIMX6L7DVN10AB and MCIMX6L8DVN10AB SoC. This
implementation assumes the value of MAJOR_UPPER (here 0x01) to be smaller
than the smallest non-6sl MAJOR_UPPER (0x61 for mx6s).

Signed-off-by: Alexander Kurz <akurz@blala.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>