of: reserved_mem: add support for using more than one region for given device

This patch allows device drivers to initialize more than one reserved
memory region assigned to given device. When driver needs to use more
than one reserved memory region, it should allocate child devices and
initialize regions by index for each of its child devices.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>