Kconfig: nand: Make MTD_NAND_FSL_ELBC depend on FSL_SOC

MTD_NAND_FSL_ELBC selects FSL_LBC that in turn depends on FSL_SOC, so
depending on PPC instead of FSL_SOC leads to this message:

	  warning: (MPC836x_RDK && MTD_NAND_FSL_ELBC &&
	  MTD_NAND_FSL_UPM) selects FSL_LBC which has unmet direct
	  dependencies (FSL_SOC)

when doing

	  make ARCH=powerpc \
	       CROSS_COMPILE=powerpc-e500v2-linux-gnuspe- \
	       allmodconfig"

Changing dependency to FSL_SOC fixes that.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>