MIPS: Alchemy: Unify Devboard support.

This patch merges support for all DB1xxx and PB1xxx
boards into a single image, along with a new single defconfig
for them.

Run-tested on DB1300 and DB1500.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Cc: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/6577/
Patchwork: https://patchwork.linux-mips.org/patch/6659/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>