fpga: fpga-mgr: make fpga_mgr_class a static const structure

fpga: fpga-mgr: make fpga_mgr_class a static const structure

Now that the driver core allows for struct class to be in read-only
memory, move the fpga_mgr_class structure to be declared at build time
placing it into read-only memory, instead of having to be dynamically
allocated at boot time.

Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Acked-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/20230811073043.52808-2-ivan.orlov0322@gmail.com
Signed-off-by: Xu Yilun <yilun.xu@intel.com>