tools: sunxi-spl-image-builder: support H6/H616 NAND boot

tools: sunxi-spl-image-builder: support H6/H616 NAND boot

The H6/H616 boot ROM doesn't expect a SPL scrambled the same way as
older SoCs.
It doesn't use a specific seeds table, it expects a maximized ECC
(BCH-80), a specific BBM (FF000301) and doesn't work if empty pages are
skipped (it needs its specific BBM, even in the padding).

So, add a --soc=h6 option to support H6/616 with:
- more ECC strengths
- specific BBM
- default_scrambler_seeds[] with all values
- no empty pages skip

In Kconfig, select BCH-80 by default for SUNXI_SPL_ECC_STRENGTH to make
BROM happy.

And in scripts/Makefile.xpl, use --soc=h6 option when building for a
SUN50I_GEN_H6 SoC.

Tested on Whatsminer H616 board, booting from NAND.

Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Co-developed-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Richard Genoud <richard.genoud@bootlin.com>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>