sunxi: h616: lower SPL stack address to avoid BROM data

When using the USB OTG FEL mode on the Allwinner H616, the BootROM
stores some data at the end of SRAM C. This is also the location where
we place the initial SPL stack, so it will overwrite this data.
We still need the BROM code after running the SPL, so should leave that
area alone.
Interestingly this does not seem to have an adverse effect, I guess on
the "way out" (when we return to FEL after the SPL has run), this data
is not needed by the BROM, for just the trailing end of the USB operation.
However this is still wrong, and we should not clobber BROM data.

Lower the SPL stack address to be situated right below the swap buffers
we use in sunxi-fel: that should be out of the way of everyone else.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Samuel Holland <samuel@sholland.org>
Tested-by: Samuel Holland <samuel@sholland.org>