device/dram: use global DIMM_SPD_SIZE Kconfig variable

Also make sure that no board changes behaviour because of that by adding
a static assert.

TEST=abuild over all builds still succeeds (where it doesn't if
DIMM_SPD_SIZE isn't set to 128 bytes for boards that use the
device/dram code).

Change-Id: Iddb962b16857ee859ddcf1b52d18da9b3be56449
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/18254
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins)

src/Kconfig

@@ -1236,7 +1236,6 @@ config DIMM_MAX
 config DIMM_SPD_SIZE
 	int
 	default 256
-	depends on GENERIC_SPD_BIN
 	help
 	  Total SPD size that will be used for DIMM.
 	  Ex: DDR3 256, DDR4 512.