nb/intel/sandybridge/raminit: Allow 933Mhz on Lenovo devices

Set max_mem_clock_mhz in devicetree to 933Mhz.
Allows to run the memory at up to DDR3-1866.

The same frequency was allowed within the first vendor bios,
but Lenovo than decided to limit it to DDR3-1333.

Tested on Lenovo T520 and DDR3-1600 DIMM (RMT3170eb86e9w16).

The RAM is now running at DDR3-1600 instead of DDR3-1333.
This gives about 4% performance increase in glmark2 using the
Intel GPU.

Change-Id: If15be497402d84a2778f0434b6381a64eda832d6
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/15158
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Martin Roth <martinroth@google.com>
8 files changed