ARM: integrator/versatile: fix NOMMU warnings

On NOMMU kernels, the io_desc variables are unused
because we don't use the MMU to remap the MMIO
areas.

Marking these variables as __maybe_unused easily
avoids the otherwise harmless warnings like

warning: 'versatile_io_desc' defined but not used

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Russell King <linux@arm.linux.org.uk>