mips: au1x00: Remove support for these SoCs

mips: au1x00: Remove support for these SoCs

The only platform left for the AU1x00 SoCs was the pb1x00 platform, an
apparent clone of the dbau1x00 platform.  As pb1x00 had no listed
maintainer I am assuming that it is also orphaned.  Remove this platform
and then remove the unused SoC support.

Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>