mtd: sunxi: Change U-Boot offset

The default U-Boot offset for the Allwinner SoCs was set to 32kB.

This was probably to try to maintain some compatibility with the current
image that we build for the MMC where the U-Boot binary is also located at
a 32kB offset.

However, this causes a number of issues. The first one is that it prevents
us from using a backup SPL entirely, which is troublesome in case where the
first would be corrupt (especially on MLC which have a higher number of
bitflips).

We also cannot use the original MMC image on the NAND, because we need to
prepare the SPL image to include the ECCs and randomizer settings, which
reduces the interest of setting it at that particular offset.

It also prevents us from upgrading and flashing the U-Boot and SPLs
independantly, since it's very likely that it will fall in the same erase
block.

Since that default wasn't used by any board, change it for 8MB, which will
be in an erase block of its own, all the erase blocks being multiple of
two. The highest erase block size we encountered is 4MB, which means that
in this particular setup, the first and second erase blocks will be for the
SPL and its backup, and the third for U-Boot.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>