pcm051: Support for revision 3

Phytec sells revision or version 3 of pcm051. It is labeled 1358.3 on
the board. The difference for u-boot is that is has other DDR3 RAM on it:
1 x MT41K256M16HA125E instead of 2 x MT41J256M8HX15E on revisions 1 and
2. Both configurations are 512 MiB.
Configure your u-boot build with pcm051_rev3 for the new RAM and
pcm051_rev1 for the old RAM configuration. Board revision 2 has to use
pcm051_rev1 also.

Signed-off-by: Lars Poeschel <poeschel@lemonage.de>