The manual documents that 0x3036006c should contains the soc revision
for imx8mq but this always reports A0. Work around this by parsing the
ROM header and checking if OCOTP register 0x40 is stuck at 0xff0055aa.

Determining this inside TF-A makes life easier for OS, see for example
this linux discussion: https://lkml.org/lkml/2019/5/3/465

The soc revision can also be useful inside TF-A itself, for example for
the non-upstream DDR DVFS "busfreq" feature is affected by 8mq erratas.

The clock for OCOTP block can be disabled by OS so only initialize soc
revision once at boot time.

Change-Id: I9ca3f27840229ce8a28b53870e44da29f63c73aa
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>