regulator: Add a pointer to aat2870_data in struct aat2870_regulator

The reason we add *pdev in struct aat2870_regulator is to use it
to get a pointer to struct aat2870_data.
Save a pointer to struct aat2870_data instead of pdev in struct
aat2870_regulator, this change makes the intention more clear.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Jin Park <jinyoungp@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>