hw: Only compile the usb-dwc2 controller if it is really needed

The USB_DWC2 switch is currently "default y", so it is included in all
qemu-system-* builds, even if it is not needed. Even worse, it does a
"select USB", so USB devices are now showing up as available on targets
that do not support USB at all. This sysbus device should only be
included by the boards that need it, i.e. by the Raspi machines.

Fixes: 153ef166 ("dwc-hsotg (dwc2) USB host controller emulation")
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Paul Zimmerman <pauldzim@gmail.com>
Message-id: 20200722154719.10130-1-thuth@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/arm/Kconfig

@@ -315,6 +315,7 @@ config RASPI
     select FRAMEBUFFER
     select PL011 # UART
     select SDHCI
+    select USB_DWC2
 
 config STM32F205_SOC
     bool

hw/usb/Kconfig

@@ -48,7 +48,6 @@ config USB_MUSB
 
 config USB_DWC2
     bool
-    default y
     select USB
 
 config TUSB6010