lib: devres: add a comment about the devm_of_iomap() function

We recently introduced a bug when we tried to convert of_iomap() to
devm_of_iomap().  The problem was that there were two drivers mapping
the same io region.  The first driver was using of_iomap() and the
second driver was using devm_of_iomap() and the kernel booted fine.
When we converted the first drive to use devm_of_iomap() then the second
driver failed with -EBUSY and the kernel couldn't boot.

Let's add a comment to prevent this sort of mistake in the future.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20200609104642.GA43074@mwanda
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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