of: Make <linux/of_reserved_mem.h> self-contained

<linux/of_reserved_mem.h> is not self-contained, as it uses
_OF_DECLARE() to define RESERVEDMEM_OF_DECLARE(), but does not include
<linux/of.h>.

Fix this by adding the missing include.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>