mtd: rawnand: davinci: Add clock resource

NAND controller has a reference clock inherited from the AEMIF
(cf. Documentation/devicetree/bindings/memory-controllers/ti-aemif.txt)
This clock isn't used yet by the driver.

Add a struct clock in the struct davinci_nand_info so it can be used
to compute timings.

Signed-off-by: Bastien Curutchet <bastien.curutchet@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>