dt-bindings: mtd: denali_dt: document reset property

According to the Denali NAND Flash Memory Controller User's Guide,
this IP has two reset signals.

  rst_n:     reset most of FFs in the controller core
  reg_rst_n: reset all FFs in the register interface, and in the
             initialization sequencer

This commit specifies these reset signals.

It is possible to control them separately from the IP point of view
although they might be often tied up together in actual SoC integration.

At least for the upstream platforms, Altera/Intel SOCFPGA and Socionext
UniPhier, the reset controller seems to provide only 1-bit control for
the NAND controller. If it is the case, the resets property should
reference to the same phandles for "nand" and "reg" resets, like this:

    resets = <&nand_rst>, <&nand_rst>;
    reset-names = "nand", "reg";

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