arm: mvebu: allow additional 4096 offset for bootable mmc image

Disarm the error message forcing u-boot/spl image to be located at
sector 0 on eMMC data-partition and microSD.
Offset 0 makes sense on eMMC boot partitions only, data partition must
use 4096 to avoid conflicting with MBR.

Valid offsets when booting from microSD, reported by boot-rom v1.73:

BootROM: Bad header at offset 00000200
BootROM: Bad header at offset 00004400
BootROM: Bad header at offset 00200000
BootROM: Bad header at offset 00400000
BootROM: Bad header at offset 00600000
BootROM: Bad header at offset 00800000
BootROM: Bad header at offset 00A00000
BootROM: Bad header at offset 00C00000
BootROM: Bad header at offset 00E00000
BootROM: Bad header at offset 01000000
BootROM: Bad header at offset 01200000
BootROM: Bad header at offset 01400000
BootROM: Bad header at offset 01600000
BootROM: Bad header at offset 01800000
BootROM: Bad header at offset 01A00000
BootROM: Bad header at offset 01C00000
BootROM: Bad header at offset 01E00000
BootROM: Bad header at offset 02000000
BootROM: Bad header at offset 02200000
BootROM: Bad header at offset 02400000
BootROM: Bad header at offset 02600000
BootROM: Bad header at offset 02800000
BootROM: Bad header at offset 02A00000
BootROM: Bad header at offset 02C00000
BootROM: Bad header at offset 02E00000

Valid offsets when booting from eMMC:

BootROM: Bad header at offset 00000000
BootROM: Bad header at offset 00200000
Switching BootPartitions.
BootROM: Bad header at offset 00000000
BootROM: Bad header at offset 00200000

Fixes: 2226ca17 ("arm: mvebu: Load U-Boot proper binary in SPL code based on kwbimage header")

Signed-off-by: Josua Mayer <josua@solid-run.com>
Reviewed-by: Stefan Roese <sr@denx.de>