device/dram/ddr3: add FTB timings

device/dram/ddr3: add FTB timings

SPD revision 1.1 introduced FTB timings, an extra set of SPD values that
specify a more precise tCKmin, tAAmin, tRCDmin, tRPmin and tRCmin.

For backwards compatibility, the MTB is usually rounded up and the FTB
part is negative. For this reason some memories were not set up optimally,
as the FTB part was ignored and the resulting timing wasn't set to the
minimum value.

The tests were performed on a Lenovo X220 with two Micron 8KTF51264HZ-1G9E
(1866 MHz): reading only the MTB part, coreboot reports a tCKmin of
1.125 ns, corresponding to a working frequency of 800 MHz; with the
additional tCKmin FTB part (-0.054 ns) the new (rounded) value is
1.070 ns, valid for a 933 MHz operation.

Tested also with Ballistix DDR3-1866 SODIMM on Lenovo T420: the memory is
now detected as DDR3-1866 instead of DDR3-1600.

Some manufacturers (like Micron) seems to expect a small rounding on the
timings, so a nearest-value rounding is performed. If this assumption
isn't correct, an error up to ~2 ps can be committed, which is low enough
to be safely ignored.

Change-Id: Ib98f2e70820f207429d04ca6421680109a81f457
Signed-off-by: Nicola Corna <nicola@corna.info>
Reviewed-on: https://review.coreboot.org/17476
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>