[POWERPC] Add powerpc PCI-E reset API implementation

Adds the pSeries platform implementation for a new PCI API
which can be used to issue various types of PCI-E reset,
including PCI-E warm reset and PCI-E hot reset. This is needed
for an ipr PCI-E adapter which does not properly implement BIST.
Running BIST on this adapter results in PCI-E errors. The only
reliable reset mechanism that exists on this hardware is PCI
Fundamental reset (warm reset).

Acked-by: Linas Vepstas <linas@austin.ibm.com>
Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>