powerpc/cell: Cleanup sysreset_hack for IBM cell blades

This patch adds a config option for the sysreset_hack used for
IBM Cell blades. The code is moves from pervasive.c into ras.c and
gets it's own init method.

Signed-off-by: Christian Krafft <krafft@de.ibm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>