mtd: rawnand: denali_dt: add more clocks based on IP datasheet

Currently, denali_dt.c requires a single anonymous clock, but
the Denali User's Guide requires three clocks for this IP:

 - clk: controller core clock

 - clk_x: bus interface clock

 - ecc_clk: clock at which ECC circuitry is run

This commit supports these named clocks to represent the real hardware.

For the backward compatibility, the driver still accepts a single clock
just as before.  The clk_x_rate is taken from the clock driver again if
the named clock "clk_x" is available.  This will happen only for future
DT, hence the existing DT files are not affected.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
Tested-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>