mtd: nand: gpmi: unify clock handling

Add device specific list of clocks required, and handle all clocks
in a single for loop. This avoids further code duplication when
adding i.MX 7 support.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>