memory: omap-gpmc: use WARN() instead of BUG() on wrong free

Since driver tracks reserved memory, freeing a non-reserved GPMC should
not be fatal and crash the system.  Printing a warning is friendlier.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200724182328.3348-9-krzk@kernel.org