ARM: mach-mvebu: handle fall-back to UART boot

The bootROM in the Armada-38x (and similar) SoC has two modes for UART
boot. The first is when the normal boot media is blank (or otherwise
missing the kwb header). The second is when the boot sequence has been
interrupted with the magic byte sequence on the UART lines.

In the first mode the bootROM routine and error code register will
indicate that there was an error booting from the configured media in
bits 7:0. In the second mode there is no error to indicate but the boot
source is provided via bits 31:28.

Handle both situations so that kwboot can be used for both boot
strapping a blank board and for intercepting a regular boot sequence.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Sean Nyekjaer <sean.nyekjaer@prevas.dk>
Signed-off-by: Stefan Roese <sr@denx.de>