MMC: au1xmmc: Remove Alchemy CPU subtype dependencies

Replace all occurrences of CONFIG_SOC_AU1??? with runtime feature detection.

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
To: Linux-MIPS <linux-mips@linux-mips.org>
Cc: linux-mmc@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/2683/
Acked-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>