mtd: rawnand: denali: remove hard-coded DENALI_DEFAULT_OOB_SKIP_BYTES

As commit 0d55c668 (mtd: rawnand: denali: set SPARE_AREA_SKIP_BYTES
register to 8 if unset") says, there were three solutions discussed:

  [1] Add a DT property to specify the skipped bytes in OOB
  [2] Associate the preferred value with compatible
  [3] Hard-code the default value in the driver

At that time, [3] was chosen because I did not have enough information
about the other platforms than UniPhier.

That commit also says "The preferred value may vary by platform. If so,
please trade up to a different solution." My intention was to replace
[3] with [2], not keep both [2] and [3].

Now that we have switched to [2] for SOCFPGA's SPARE_AREA_SKIP_BYTES=2,
[3] should be removed. This should be OK because denali_pci.c just
gets back to the original behavior.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>