mtd: nand: add common DT init code

These are already-documented common bindings for NAND chips. Let's
handle them in nand_base.

If NAND controller drivers need to act on this data before bringing up
the NAND chip (e.g., fill out ECC callback functions, change HW modes,
etc.), then they can do so between calling nand_scan_ident() and
nand_scan_tail().

The original commit has been slightly reworked to use the fdtdec_xxx()
helpers (instead of the of_xxxx() ones).

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>