fsi: scom: Major overhaul

This was too hard to split ... this adds a number of features
to the SCOM user interface:

 - Support for indirect SCOMs

 - read()/write() interface now handle errors and retries

 - New ioctl() "raw" interface for use by debuggers

Signed-off-by: Benjamin Herrenschmidt <[email protected]>
Reviewed-by: Eddie James <[email protected]>
Reviewed-by: Alistair Popple <[email protected]>

Author: ozbenh