venus: core: Parse firmware-name DT property

On production devices the firmware could be located on different
places, this path could be provided by special firmware-name DT
property.

Here we check for existence of such DT property and if it exist
take the firmware path from there. Otherwise, if the property
is missing we fallback to the predefined path from driver resource
structure.

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Reviewed-By: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>