powerpc/fsl: Remove FSL_SOC dependency from FSL_LBC

This dependency led to kconfig errors when MTD_NAND_FSL_ELBC was
enabled, which selects FSL_LBC, in the absence of FSL_SOC, as reported
in http://patchwork.ozlabs.org/patch/564405/

It was originally suggested to add an FSL_SOC dependency to
MTD_NAND_FSL_ELBC, but the FSL_SOC symbol has been a growing problem
due to hardware being shared between PPC and ARM SoCs.  Even though
eLBC isn't found on ARM SoCs (the newer IFC is used instead), I don't
want to expand the use of FSL_SOC for things other than functions
exported by fsl_soc.c.  In particular, it would be odd to add it to
MTD_NAND_FSL_ELBC and then remove it from MTD_NAND_FSL_IFC.

Removing artificial dependencies also helps get compile-test exposure
via randconfig, allyesconfig, etc.

Reported-by: Brian Norris <computersforpeace@gmail.com>
Cc: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Scott Wood <oss@buserror.net>