i2c: at91: mark PM ops as __maybe unused

The driver uses pm_ptr(), so the PM ops could be unused.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: dab4b0e8 ("i2c: at91: remove #define CONFIG_PM")
Signed-off-by: Wolfram Sang <wsa@kernel.org>
Reviewed-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>