rtc: max77686: Report platform modalias to fix module autoload

If the rtc-max77686 driver is built as a module, modalias information is
not filled so the module is not autoloaded. Use the MODULE_DEVICE_TABLE()
macro to export the platform ID table so the module contains that data.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>