nxp: Finish migration of SYS_FSL_IFC_BANK_COUNT to Kconfig

As this is used on both ARM and PowerPC platforms, this needs to be
asked in arch/Kconfig.nxp.  Set the PowerPC defaults based on
arch/powerpc/include/asm/config_mpc85xx.h and remove the default set in
drivers/mtd/nand/raw/fsl_ifc_nand.c

Signed-off-by: Tom Rini <trini@konsulko.com>