of: document bindings for reserved-memory nodes

Reserved memory nodes allow for the reservation of static (fixed
address) regions, or dynamically allocated regions for a specific
purpose.

[joshc: Based on binding document proposed (in non-patch form) here:
 http://lkml.kernel.org/g/20131030134702.19B57C402A0@trevor.secretlab.ca
 adapted to support #memory-region-cells]
Signed-off-by: Josh Cartwright <joshc@codeaurora.org>
[mszyprow: removed #memory-region-cells property]
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
[grant.likely: removed residual #memory-region-cells example]
Signed-off-by: Grant Likely <grant.likely@linaro.org>