mtd: rawnand: gpmi: use runtime PM to manage clocks

The gpmi driver aggressively en/disables the clocks between operations
which has significant performance cost. Use runtime PM to get rid of
this bottleneck.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>