staging: greybus: enable compile testing of arche driver

Add Arche platform-driver config option and allow the driver to be
compile tested also without the out-of-tree usb3613 driver.

Signed-off-by: Johan Hovold <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/greybus/Kconfig

···
 	  will be called gb-usb.ko
 
 endif	# GREYBUS_BRIDGED_PHY
+
+config GREYBUS_ARCHE
+	tristate "Greybus Arche Platform driver"
+	depends on USB_HSIC_USB3613 || COMPILE_TEST
+	---help---
+	  Select this option if you have an Arche device.
+
+	  To compile this code as a module, chose M here: the module
+	  will be called gb-arche.ko
+
 endif	# GREYBUS

drivers/staging/greybus/Makefile

···
 # Greybus Platform driver
 gb-arche-y	:= arche-platform.o arche-apb-ctrl.o
 
-obj-$(CONFIG_USB_HSIC_USB3613)	+= gb-arche.o
+obj-$(CONFIG_GREYBUS_ARCHE)	+= gb-arche.o

drivers/staging/greybus/arche-platform.c

···
 #include "arche_platform.h"
 #include "greybus.h"
 
+#if IS_ENABLED(CONFIG_USB_HSIC_USB3613)
 #include <linux/usb/usb3613.h>
+#else
+static inline int usb3613_hub_mode_ctrl(bool unused)
+{
+	return 0;
+}
+#endif
 
 #define WD_COLDBOOT_PULSE_WIDTH_MS	30