ARC: devboards: Allow huge uImages (up to 128 MiB)

Even though in production uImage usually is quite small as
it contains just Linux kernel image during development it might
be pretty convenient to have root-FS built into the same image.

That makes uImage much larger but given on our dev platforms we have
quite a lot of DDR (> 512 MiB) we may afford loading huge uImages.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>