POST: add new memory regions test

This test is similar to the actual POST memory test but quicker and
far less complete. It checks the address and data lines and then only
tests some regularly placed sub regions of the RAM.
This can be useful when we want to test the RAM but we do not have enough
time to run the full memory test.

The POST memory test code was rearranged in order to avoid code duplication
between the two tests but the memory test functionnality remains the same.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Ackey-by: Mike Frysinger <vapier@gentoo.org>