ARM: davinci: dm365 evm: fix unused variable warning

If neither CONFIG_SND_DM365_AIC3X_CODEC nor CONFIG_SND_DM365_VOICE_CODEC
are defined, we may get warnings like:

    arch/arm/mach-davinci/board-dm365-evm.c:179:33: warning: 'dm365_evm_snd_data' defined but not used [-Wunused-variable]

To fix this, just mark the struct as __maybe_unused.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Sekhar Nori <nsekhar@ti.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>