mtd: rawnand: sunxi: do not count BBM bytes twice

mtd: rawnand: sunxi: do not count BBM bytes twice

BBM is already part of USER_DATA section, so we should not remove it twice

This was working ok because we are on the safe size, advertising that
there was 2 bytes less available than in reality.

But we can't change old platforms, since it may lead to a different ECC
strength, so, introduce a legacy flag for old platforms, and switch the
new platforms to the correct count.

Signed-off-by: Richard Genoud <richard.genoud@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>