UPSTREAM: of/overlay: Directly include idr.h

The overlay code uses IDRs but does not explicitly include the header
providing the interface, instead relying on an implicit inclusion. Make
the dependency explicit to avoid potential future build issues if the
implicit inclusion goes away.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
(cherry picked from commit 0d1886df6f770f881daa275fc2d3aae0c1cb5c9d)
Bug: 109890997
Change-Id: I1756b2fd3b4a7ace7359add9fc87c21edc1fcd47
Signed-off-by: John Stultz <john.stultz@linaro.org>