fpga: Add Kconfig to fpga subsystem

Add missing Kconfig to fpga subsystem to be able
to add new options.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/drivers/fpga/Kconfig b/drivers/fpga/Kconfig
new file mode 100644
index 0000000..1ba02f0
--- /dev/null
+++ b/drivers/fpga/Kconfig
@@ -0,0 +1,3 @@
+menu "FPGA support"
+
+endmenu