firmware: Add FIRMWARE config prompt string

There is no prompt string for FIRMWARE config. Without this,
FIRMWARE config cannot be enabled through menuconfing or
config file. Fix this by adding prompt summary.

Signed-off-by: Rajan Vaja <rajan.vaja@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

drivers/firmware/Kconfig

@@ -1,5 +1,5 @@
 config FIRMWARE
-	bool
+	bool "Enable Firmware driver support"
 
 config ARM_PSCI_FW
 	bool