[POWERPC] maple: improve CPC9x5 host bridge detection

Identify CPC9x5 PCI Express, AGP, and HT host bridges using
device_type and compatible properties, which is a more flexible method
than using the name property (which can differ between firmwares and
models).

Signed-off-by: Nathan Lynch <ntl@pobox.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>