MIPS: Alchemy: db1200: Improve PB1200 detection.

MIPS: Alchemy: db1200: Improve PB1200 detection.

The PB1200 has the CPLD located at an address which on the DB1200 is
RAM;  reading the Board-ID sometimes results in a PB1200 being detected
instead (especially during reboots after long uptimes).
On the other hand, the address of the DB1200's CPLD is hosting Flash
chips on the PB1200.  Test for the DB1200 first and additionally do a
quick write-test to the hexleds register to make sure we're writing
to the CPLD.

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3005/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>