ath79: prepare NAND subtarget for upstream support of SPI NAND

ath79: prepare NAND subtarget for upstream support of SPI NAND

Linux 4.19 supplies the upstream spi-nand framework,
permitting porting and support of boards with SPI NAND.

  * Adjusted nand/target.mk to provide FEATURES += squashfs nand

  * Updated config-default to provide current MTD and UBI support

Defaults selected for:

  CONFIG_MTD_UBI_WL_THRESHOLD=4096
  CONFIG_MTD_UBI_BEB_LIMIT=20
  # CONFIG_MTD_UBI_FASTMAP is not set
  # CONFIG_MTD_UBI_GLUEBI is not set

The bad-block reservation limit technically should be 21 for Paragon
SPI NAND but most other devices in the class are 20 blocks per Gbit.
In Linux 5.2 this is specified on a per-chip basis through NAND_MEMORG

Contents adjusted to remove declarations provided at the target level
by commit 08a134820f5 (Oct 23, 2019) ath79: enable PCI for whole target

Signed-off-by: Jeff Kletsky <git-commits@allycomm.com>
Tested-by: Michal Cieslakiewicz <michal.cieslakiewicz@wp.pl>