cmake: extensions: add NAME parameter to DT register helpers

Allow register address and size access by name.

Example devicetree fragment:

  / {
          foo@deadbeef {
                  reg = <0xdeadbeef 0x1000>, <0xfeedface 0x2000>;
                  reg-names = "bar", "baz";
          };
  };

Example usage:

  dt_reg_addr(bar_addr PATH "/foo@deadbeef" NAME bar)
  dt_reg_size(bar_size PATH "/foo@deadbeef" NAME bar)

  dt_reg_addr(baz_addr PATH "/foo@deadbeef" NAME baz)
  dt_reg_size(baz_size PATH "/foo@deadbeef" NAME baz)

Results:

  - bar_addr is 0xdeadbeef
  - bar_size is 0x1000
  - baz_addr is 0xfeedface
  - baz_size is 0x2000

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>