memory: atmel-ebi: mark PM ops as __maybe_unused

We get a harmless warning without CONFIG_PM:

drivers/memory/atmel-ebi.c:584:12: error: 'atmel_ebi_resume' defined but not used [-Werror=unused-function]

Marking the function as __maybe_unused does the right thing here
and drops it silently when unused.

Fixes: a483fb10e5ea ("memory: atmel-ebi: Add PM ops")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>