mtd: sm_ftl: remove CONFIG_SM_FTL_MUSEUM and make it always on

The sole purpose of this setting was to avoid a dependency on MTD_NAND.
Now that we can depend on MTD_NAND_ECC without pulling in all the rest
of the NAND code, we might as well do so unconditionally.

Signed-off-by: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>