time: tick: implemeted a sys_tick define

sys_tick:
	The CPP will process it into atomic64_get(&sys_clk->tc). This
	the sys_tick can be used as an atomic variable.

time_after/time_before
	Define function to calculate if a certain amount of time has
	passed (or not).

Signed-off-by: Michel Megens <dev@michelmegens.net>