The current code doesn't check whether platform_get_resource_byname()
succeeded to get the l1tcm memory, which is optional, before attempting
to map it. This results in the following error message when it is
missing:

  mtk-scp 10500000.scp: error -EINVAL: invalid resource (null)

Add a check so that the remapping is only attempted if the memory region
exists. This also allows to simplify the logic handling failure to
remap, since a failure then is always a failure.

Fixes: ca23ecfd ("remoteproc/mediatek: support L1TCM")
Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reviewed-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20240627-scp-invalid-resource-l1tcm-v1-1-7d221e6c495a@collabora.comSigned-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>