remoteproc: mtk_scp: Construct FW path if firmware-name not present

After a reply on the mailing lists [1] it emerged that the DT
property "firmware-name" should not be relied on because of
possible issues with firmware versions.
For MediaTek SCP, there has never been any firmware version vs
driver version desync issue but, regardless, the firmwares are
always using the same name and they're always located in a path
with a specific pattern.

Instead of unconditionally always relying on the firmware-name
devicetree property to get a path to the SCP FW file, drivers
should construct a name based on what firmware it knows and
what hardware it is running on.

In order to do that, add a `scp_get_default_fw_path()` function
that constructs the path and filename based on two of the infos
that the driver can get:
 1. The compatible string with the highest priority (so, the
    first one at index 0); and
 2. The type of SCP HW - single-core or multi-core.

This means that the default firmware path is generated as:
 - Single core SCP: mediatek/(soc_model)/scp.img
   for example:     mediatek/mt8183/scp.img;

 - Multi core SCP:  mediatek/(soc_model)/scp_c(core_number).img
   for example:     mediatek/mt8188/scp_c0.img for Core 0, and
                    mediatek/mt8188/scp_c1.img for Core 1.

Note that the generated firmware path is being used only if the
"firmware-name" devicetree property is not present in the SCP
node or in the SCP Core node(s).

[1 - Reply regarding firmware-name property]
Link: https://lore.kernel.org/all/7e8718b0-df78-44a6-a102-89529d6abcce@app.fastmail.com/
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20251015084103.10737-1-angelogioacchino.delregno@collabora.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>

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