pdbg: Move to v3.0

The 3.0 release follows from the 2.5-rc1. The changelog for 3.0 is as
follows:

Major upstream change was the split out the access method from the
device-tree representation to enable backend agnostic system device
trees.

Other notable features:

 - Swift support
 - get/putmempba
 - Added SBEFIFO backend
 - Expanded use of SBEFIFO chipops

(From meta-openpower rev: 9c2e7744375b992d4cc7cca91d9dc9009f2b76e9)

Signed-off-by: Joel Stanley <joel@jms.id.au>
Change-Id: I3a1ed00c0b3bacc64312fe6c09b4b5b13e691772
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>