openrisc-timer: Reduce overhead, Separate clock update functions

The clock value is only evaluated when really necessary reducing
the overhead of the timer handling.

This also solves a problem in the way the Linux kernel
handles the timer and the expected accuracy.
The old version could lead to inaccurate timings.

Signed-off-by: Sebastian Macke <sebastian@macke.de>
Reviewed-by: Jia Liu <proljc@gmail.com>
Signed-off-by: Jia Liu <proljc@gmail.com>