of/platform: Generalize /reserved-memory handling

By iterating over all /reserved-memory child nodes and match each one to
a list of compatibles that we want to treat specially, we can easily
extend the list of compatibles to handle - without having to resort to
of_platform_populate() that would create unnecessary platform_devices.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>