arm64: mte: Update "block-size" field specification for tag storage

Change the "block-size" field from the metadata node to represent the
number of pages in a tag storage block, instead of the size in bytes.  This
will remove the ambiguity in describing block sizes larger than one page.

With this change, the node becomes:

    metadata1: metadata@8c0000000  {
            compatible = "arm,mte-tag-storage";
            reg = <0x08 0xc0000000 0x00 0x1000000>;
            block-size = <1>;  // In pages
            memory = <&memory1>; // Associated tagged memory
    };