mtd: rawnand: denali: set SPARE_AREA_SKIP_BYTES register to 8 if unset

mtd: rawnand: denali: set SPARE_AREA_SKIP_BYTES register to 8 if unset

NAND devices need additional data area (OOB) for error correction,
but it is also used for Bad Block Marker (BBM).  In many cases, the
first byte in OOB is used for BBM, but the location actually depends
on chip vendors.  The NAND controller should preserve the precious
BBM to keep track of bad blocks.

In Denali IP, the SPARE_AREA_SKIP_BYTES register is used to specify
the number of bytes to skip from the start of OOB.  The ECC engine
will automatically skip the specified number of bytes when it gets
access to OOB area.

The same value for SPARE_AREA_SKIP_BYTES should be used between
firmware and the operating system if you intend to use the NAND
device across the control hand-off.

In fact, the current denali.c code expects firmware to have already
set the SPARE_AREA_SKIP_BYTES register, then reads the value out.

If no firmware (or bootloader) has initialized the controller, the
register value is zero, which is the default after power-on-reset.
In other words, the Linux driver cannot initialize the controller
by itself.

Some possible solutions are:

 [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

My first attempt was [1], but in the review process, [3] was suggested
as a counter-implementation.
(https://lore.kernel.org/patchwork/patch/983055/)

The default value 8 was chosen to match to the boot ROM of the UniPhier
platform.  The preferred value may vary by platform.  If so, please
trade up to a different solution.

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