ppc4xx: IBM Memory Controller DDR autocalibration routines

Alternate SDRAM DDR autocalibration routine that can be generically used
for any PPC4xx chips that have the IBM SDRAM Controller core allowing for
support of more DIMM/memory chip vendors and gets the DDR autocalibration
values which give the best read latency performance (SDRAM0_RDCC.[RDSS]).

Two alternate SDRAM DDR autocalibration algoritm are provided in this patch,
"Method_A" and "Method_B".  DDR autocalibration Method_A scans the full range
of possible PPC4xx  SDRAM Controller DDR autocalibration values and takes a
lot longer to run than Method_B.  Method_B executes in the same amount of time
as the currently existing DDR autocalibration routine, i.e. 1 second or so.
Normally Method_B is used and it is set as the default method.

The current U-Boot PPC4xx DDR autocalibration code calibrates the IBM SDRAM
Controller registers.[bit-field]:
1)  SDRAM0_RQDC.[RQFD]
2)  SDRAM0_RFDC.[RFFD]

This alternate PPC4xx DDR autocalibration code calibrates the following
IBM SDRAM Controller registers.[bit-field]:

1)  SDRAM0_WRDTR.[WDTR]
2)  SDRAM0_CLKTR.[CKTR]
3)  SDRAM0_RQDC.[RQFD]
4)  SDRAM0_RFDC.[RFFD]

and will also use the calibrated settings of the above four registers that
produce the best "Read Sample Cycle Select" value in the SDRAM0_RDCC.[RDSS]
register.[bit-field].

Signed-off-by: Adam Graham <agraham@amcc.com>
Signed-off-by: Stefan Roese <sr@denx.de>