tools: mksunxiboot: allow larger SPL binaries

mksunxiboot limits the size of the resulting SPL binaries to pretty
conservative values to cover all SoCs and all boot media (NAND).
It turns out that we have limit checks in place in the build process,
so mksunxiboot can be relaxed and allow packaging binaries up to the
actual 32KB the mask boot ROM actually imposes.
This allows to have a bigger SPL, which is crucial for AArch64 builds.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@openedev.com>