fpga: Add Kconfig to fpga subsystem

Add missing Kconfig to fpga subsystem to be able
to add new options.
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>

drivers/Kconfig

@@ -20,6 +20,8 @@ source "drivers/dfu/Kconfig"
 
 source "drivers/dma/Kconfig"
 
+source "drivers/fpga/Kconfig"
+
 source "drivers/gpio/Kconfig"
 
 source "drivers/hwmon/Kconfig"

drivers/fpga/Kconfig

+menu "FPGA support"
+
+endmenu