mach-u300: retire odd singlemem variant

The U300 had a special variant where a memory was shared
between two CPU:s and only one part of memory was
accessible for the kernel. However it is now seriously
legacy and only create problems for kernel consolidation,
so let's retire it.

Cc: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>