i2c: at91: remove #define CONFIG_PM

Remove #define CONFIG_PM and use __maybe_unused for PM functions and
pm_ptr() for PM ops.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Reviewed-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>