dt-binding: mtd: denali_dt: document clock property

Commit 30f9f2fb ("mtd: denali: add a DT driver") supported the
clock enablement, but did not document it in the DT binding.

In addition to the existing clock, this commit adds more clocks based
on the IP specification.

According to the Denali User's Guide, this IP needs three clocks:

 - clk: controller core clock

 - clk_x: bus interface clock

 - ecc_clk: clock at which ECC circuitry is run

The driver should accept the current single clock for the backward
compatibility, but the DT binding should represent the real hardware,
and future platforms must follow this.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>