sunxi: spl: h616: fix booting from high MMC offset

sunxi: spl: h616: fix booting from high MMC offset

The BootROM in the Allwinner H616 tries to load the initial boot code
from sector 16 (8KB) of an SD card or eMMC device, but also looks at
sector 512 (256KB). This helps with GPT formatted cards.
A "high" boot offset is also used on previous SoCs, but it's sector 256
(128KB) there instead.

Extend the existing offset calculation code to consider the different
sector offset when running on an H616 SoC. This allows to load U-Boot
on any H616 device when the SPL is not located at 8KB.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Tested-by: Ryan Walklin <ryan@testtoast.com>