mtd: rawnand: denali: configure SPARE_AREA_SKIP_BYTES only for denali_spl

This CONFIG option is only used in denali_spl.c

Move it close to SPL_NAND_DENALI, and make it depend on SPL_NAND_DENALI.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>