basic_mmio_gpio: convert to platform_{get,set}_drvdata()

Use the platform drvdata helpers rather than working on the struct
device itself.

Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Acked-by: Anton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>