sunxi: Automate loading from 128KB MMC offset

Since commit 067e0b96 ("sunxi: Allow booting from 128KB SD/eMMC offset")
we support having the SPL loaded from either the traditional 8KB SD
card/eMMC offset, or from the alternative location at 128KB. However the
sector to find the U-Boot image was still hard-coded at compile time,
and had to be adjusted for one of the two choices.

Since we can actually override the function to return the sector offset,
we can just check the boot source byte there to select the proper offset
based on from where the SPL was loaded.

This allows the very same binary image to be loaded from either 128KB or
8KB, with the U-Boot proper image always being located just behind the SPL.

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