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: Arnd Bergmann <arnd@arndb.de>