sunxi: support loading with SPL > 32KB

H616 supports and needs bigger SPL than 32 KiB, mostly due to big DRAM
driver and need for PMIC configuration, which pulls several drivers which
are not needed otherwise.

spl_mmc_get_uboot_raw_sector() will now compare pre-configured size with
that, reported in SPL header. If size in header is bigger, it will use
that value instead.

In the process of function rework, also add missing function argument.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Samuel Holland <samuel@sholland.org>