Allow to extend expression of sub nodes to optionally indicate bit
position. This extension is needed to distinguish between different bit
positions in the same address.

For example, there are two nvmem nodes starting with bit 4 and bit 0
at the same address 0x54. In this case, it can be expressed as follows.

    trim@54,4 {
        reg = <0x54 1>;
        bits = <4 2>;
    };
    trim@54,0 {
        reg = <0x54 1>;
        bits = <0 4>;
    };
Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Link: https://lore.kernel.org/r/1626661864-15473-2-git-send-email-hayashi.kunihiko@socionext.comSigned-off-by: Rob Herring <robh@kernel.org>