Combine support for the DB1200/PB1200, DB1300 and DB1550 boards into
a single kernel image.

defconfig-generated image verified on DB1200, DB1300 and DB1550.
Signed-off-by: NManuel Lauss <manuel.lauss@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/4335/Signed-off-by: NRalf Baechle <ralf@linux-mips.org>