arm: mvebu: fix boot from UART on ClearFog Base

The ClearFog Base boot from UART when setting the DIP switches to 01001.
Unfortunately, the SPL code sometimes fails to detect the UART boot
method at run-time. Add an alternative SAR UART boot value to fix this.

Note that this alternative value is not documented (Armada 38x Hardware
Specifications, Table 48). But experimentations showed it on the
ClearFog Base.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Stefan Roese <sr@denx.de>