nvmem: core: allow specifying of_node

Until now, the of_node of the parent device is used. Some devices
provide more than just the nvmem provider. To avoid name space clashes,
add a way to allow specifying the nvmem cells in subnodes. Consider the
following example:

    flash@0 {
        compatible = "jedec,spi-nor";

        partitions {
            compatible = "fixed-partitions";
            #address-cells = <1>;
            #size-cells = <1>;

            partition@0 {
                reg = <0x000000 0x010000>;
            };
        };

        otp {
            compatible = "user-otp";
            #address-cells = <1>;
            #size-cells = <1>;

            serial-number@0 {
                reg = <0x0 0x8>;
            };
        };
    };

There the nvmem provider might be the MTD partition or the OTP region of
the flash.

Add a new config->of_node parameter, which if set, will be used instead
of the parent's of_node.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210424110608.15748-2-michael@walle.cc