MIPS: Alchemy: remove all CONFIG_SOC_AU1??? defines

Now that no driver any longer depends on the CONFIG_SOC_AU1???  symbols,
it's time to get rid of them: Move some of the platform devices to the
boards which can use them, Rename a few (unused) constants in the header,
Replace them with MIPS_ALCHEMY in the various Kconfig files.  Finally
delete them altogether from the Alchemy Kconfig file.

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
To: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/2707/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>