powerpc/85xx: implement check for erratum A-004849 work-around

The work-around for erratum A-004849 ("CoreNet fabric (CCF) can exhibit a
deadlock under certain traffic patterns causing the system to hang") is
implemented via the PBI (pre-boot initialization code, typically attached
to the RCW binary).  This is because the work-around is easier to implement
in PBI than in U-Boot itself.

It is still useful, however, for the 'errata' command to tell us whether
the work-around has been applied.  For A-004849, we can do this by verifying
that the values in the specific registers that the work-around says to
update.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>