ARM: mmp: mark usb_dma_mask as __maybe_unused

This variable may be used by some devices that each have their
on Kconfig symbol, or by none of them, and that causes a build
warning:

arch/arm/mach-mmp/devices.c:241:12: error: 'usb_dma_mask' defined but not used [-Werror=unused-variable]

Marking it __maybe_unused avoids the warning.
Signed-off-by: NArnd Bergmann <arnd@arndb.de>

arch/arm/mach-mmp/devices.c

@@ -238,7 +238,7 @@ void pxa_usb_phy_deinit(void __iomem *phy_reg)
 #endif
 
 #if IS_ENABLED(CONFIG_USB_SUPPORT)
-static u64 usb_dma_mask = ~(u32)0;
+static u64 __maybe_unused usb_dma_mask = ~(u32)0;
 
 #if IS_ENABLED(CONFIG_USB_MV_UDC)
 struct resource pxa168_u2o_resources[] = {