arm: Add devicetree fixup machine function

Commit 1c2f87c2
(ARM: 8025/1: Get rid of meminfo) dropped the upper bound on
the number of memory banks that can be added as there was no
technical need in the kernel. It turns out though, some bootloaders
(specifically the arndale-octa exynos boards) may pass invalid memory
information and rely on the kernel to not parse this data. This is a
bug in the bootloader but we still need to work around this.
Work around this by introducing a dt_fixup function. This function
gets called before the flattened devicetree is scanned for memory
and the like. In this fixup function for exynos, limit the maximum
number of memory regions in the devicetree.

Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Tested-by: Andreas Färber <afaerber@suse.de>
[glikely: Added a comment and fixed up function name]
Signed-off-by: Grant Likely <grant.likely@linaro.org>