clkcnt: Add clockcount abstraction.

This commit added the clkcnt abstraction.
Clkcnt is used to abstract the cycle counter of hardware timer, which can represent almost all existing timers. Time conversion through API provided by clkcnt can avoid any timing conversion problem (multiplication overflow, division precision loss, etc.).

Signed-off-by: ouyangxiangzhen <[email protected]>

Author: Fix-Point