Rename timer tick implementations to tick_*.c to be consistent with
msp430 (clock_*.c there is not tick implementation, but general
oscillator / clock setup).
Move timer tick implementation into separate file.
Add watchdog timer based tick initial implementation.
Add clock_aclk.h to deal with different ACLK sources.
Rename clock_aclk.h to port_aclk.h, now port header files are named like
port*.h
Select WDT timer settings based on HZ value.