HSD #14015013554: fpga: stratix10-soc: Support multi-client multi-threading

commit debbf83287e63972e18779305b8b5235116f5f9b from
https://github.com/altera-opensource/linux-socfpga.git

 socfpga-5.15.30-lts.

This minor fix is to ensure, that the FPGA driver releases the SVC mailbox
resource after usage.
The change also ensures, that the FPGA driver is able to function properly
when run together with another SVC driver like soc64-hwmon and intel_fcs.

Signed-off-by: Ang Tien Sung <tien.sung.ang@intel.com>
Signed-off-by: Liwei Song <liwei.song@windriver.com>