env: sunxi: Define location in SPI flash

To allow loading and storing the environment from SPI flash, adjust the
raw offset variables for Allwinner boards to make sense there.

U-Boot (including SPL and other blobs) is loaded from the beginning of
SPI flash, so move the environment location as far back as possible, to
not create unnecessary limits. As those offsets are shared with (now
mostly unused) raw MMC environment, we should respect the common one
megabyte limit, which also makes sense on SPI flash.

So limit the environment for those raw locations to 64KB, and place it
just below 1MB (@960KB).

Those values are currently unused, unless someone forcibly enables the
raw MMC environment. In this case it would break as of now, as the
current offset of 544KB is far too low for the current (arm64) U-Boot
proper.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>