sunxi: Allow booting from 128KB SD/eMMC offset

On modern Allwinner SoCs (tested: H2+, A64, H5, H6) the BootROM can
actually load the SPL also from sector 256 (128KB) of an SD card or eMMC
chip. For more details, see [1].
In this case the boot source indicator (written at offset 0x28 of SRAM A1)
has bit 4 set, so it's 0x10 for SD card and 0x12 for eMMC.

Add those new values to the existing boot source check to allow booting
the SPL from those "high" disk offsets as well. For this to work, the
value of CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR needs to be adjusted,
for instance to 0x140 (right after the high SPL). Doing this dynamically
sounds desirable, but looks nasty to implement.

[1] https://groups.google.com/forum/#!topic/linux-sunxi/MaiijyaAFjk



Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>