Blackfin arch: cache SWRST value at bootup so other things like watchdog can...

Blackfin arch: cache SWRST value at bootup so other things like watchdog can non-destructively query it

Signed-off-by: Mike Frysinger <michael.frysinger@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>