FDT: Add fixup support for multiple banks of memory

Add fdt_fixup_memory_banks and reimplement fdt_fixup_memory
using it.  Tested on OMAP3 beagle board with two banks of
memory.

Signed-off-by: John Rigby <john.rigby@linaro.org>
CC: Jerry Van Baren <vanbaren@cideas.com>
Acked-by: Gerald Van Baren <vanbaren@cideas.com>