clocksource: sh_tmu: Replace global spinlock with a per-device spinlock

The global spinlock is used to protect the shared start/stop register.
Now that all TMU channels are handled by a single device instance, use a
per-device spinlock.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Simon Horman <horms+renesas@verge.net.au>