rtc: add support for spi rtc rx4581

[akpm@linux-foundation.org: checkpatch fixes]
Signed-off-by: Torben Hohn <torbenh@linutronix.de>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>