sbc8548: Make enabling SPD RAM configuration work

Previously, SPD configuration of RAM was non functional on
this board.  Now that the root cause is known (an i2c address
conflict), there is a simple end-user workaround - remove the
old slower local bus 128MB module and then SPD detection on the
main DDR2 memory module works fine.

We make the enablement of the LBC SDRAM support conditional on
being not SPD enabled.  We can revisit this dependency as the
hardware workaround becomes available.

Turning off LBC SDRAM support revealed a couple implict dependencies
in the tlb/law code that always expected an LBC SDRAM address.

This has been tested with the default 256MB module, a 512MB
a 1GB and a 2GB, of varying speeds, and the SPD autoconfiguration
worked fine in all cases.

The default configuration remains to go with the hard coded
DDR config, so the default build will continue to work on boards
where people don't bother to read the docs.  But the advantage
of going to the SPD config is that even the small default module
gets configured for CL3 instead of CL4.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>