of: reserved-memory: Add stub for RESERVEDMEM_OF_DECLARE()

The reserved-memory Kconfig could be disabled when drivers are
compile-tested. In this case RESERVEDMEM_OF_DECLARE() produces a
noisy warning about the orphaned __reservedmem_of_table section.
Add the missing stub that fixes the warning. In particular this is
needed for compile-testing of NVIDIA Tegra210 memory driver which
uses reserved-memory.

Reported-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20210610162313.20942-1-digetx@gmail.com


Signed-off-by: Rob Herring <robh@kernel.org>