drivers/rtc/rtc-imxdi.c: must include <linux/spinlock.h>

Add the missing header include for spinlocks, to avoid potential build
failures on specific architectures or configurations.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>