arm: mvebu: db-88f6820-amc: Add defconfig for NAND booting

arm: mvebu: db-88f6820-amc: Add defconfig for NAND booting

This new db-88f6820-amc_nand_defconfig file is copy of existing
db-88f6820-amc_defconfig file and changed to instruct build system to
generate final kwbimage for NAND booting. It was done by adding options:

    CONFIG_MVEBU_SPL_BOOT_DEVICE_NAND=y
    CONFIG_MVEBU_SPL_NAND_BADBLK_LOCATION=0x00
    CONFIG_SYS_NAND_BLOCK_SIZE=0x40000
    CONFIG_SYS_NAND_PAGE_SIZE=0x1000

Board has Micron MT29F8G08ABACAWP chip which is SLC NAND with 4kB page size
and block size of 64 pages.

This change was only compile-tested and is useful for CI testing that
mkimage can generate valid kwbimage of NAND type.

This change is more readable via git option --find-copies-harder.

Signed-off-by: Pali Rohár <pali@kernel.org>