powerpc/eeh: pseries platform EEH error log retrieval

On RTAS compliant pSeries platform, one dedicated RTAS call has
been introduced to retrieve EEH temporary or permanent error log.

The patch implements the function of retriving EEH error log through
RTAS call. Besides, it has been abstracted by struct eeh_ops::get_log
so that EEH core components could support multiple platforms in future.

Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>