[dev][ram-nand] Add support for attaching metadata to spawned device.

This enables spawning of devices which the nandpart and skip-block
drivers can attach to.

Change-Id: I84a346c44580f4e313655dc3f680592e04077d75
Tested: Ran `runtests -t ram-nand-test`

[ram-nand] Add faked ddk for unit tests.

This CL removes a wrapper around the DDKTL device implementation and
replaces that with a trivial fake of the ddk for the unit test.

Test: ram-nand-test
Change-Id: I33ee28cfb6fa759fdd6433dfb64030906482e3a5