fpga: add fpga_compatible2flag

Add a "compatible" string to binary flag converter, which uses
a callback str2flag() of given FPGA driver if available.

Signed-off-by: Oleksandr Suvorov <oleksandr.suvorov@foundries.io>
Tested-by: Ricardo Salveti <ricardo@foundries.io>
Tested-by: Adrian Fiergolski <adrian.fiergolski@fastree3d.com>
Link: https://lore.kernel.org/r/20220722141614.297383-8-oleksandr.suvorov@foundries.io


Signed-off-by: Michal Simek <michal.simek@amd.com>