[coreboot] [PATCH]Refactor SPDROM access

[Patrick Georgi]

Hi,

attached patch cleans up SPDROM access and gets rid of the DIMMx
definitions in romstage.cs (in favor of a single set of them in spd.h).

Of special note is that amd/cs5535 and amd/cs5536 used different values
for DIMMx: These were simply bitshifted. Their only use is in
smbus_read_byte (pre-ram variant, which doesn't seem to be used for
anything else), so I adapted their version to do the same bitshift the
other implementations do. That way, they can reuse the same DIMMx
defines, too.

A later change would be to move the 90% of spd_read_byte implementations
in the boards to a unified one (weak function) that can be overridden by
boards that really behave differently (eg. those with fixed RAM without
SPD ROMs). The single-DIMM special case could be accomodated with a
Kconfig variable then.

The patch is untested, so please review carefully.

Signed-off-by: Patrick Georgi