MIPS: Alchemy: merge PB1550 support into DB1550 code

MIPS: Alchemy: merge PB1550 support into DB1550 code

The PB1550 is more or less a DB1550 without the PCI IDE controller,
a more complicated (read: configurable) Flash setup and some other
minor changes.  Like the DB1550 it can be automatically detected by
reading the CPLD ID register bits.

This patch adds PB1550 detection and setup to the DB1550 code.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/4337/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>