The attiny and other microcontrollers use their crystal controlled clock to generate their PWM signal. They are therefore restricted to producing a maximum signal that is at least half as much as their clock frequency.
With 20Mhz crystal controlled micro controller and software signal generator you might be able to generate from 0 to 10Mhz ( as mentioned by johnwasser)
I see two ways to do what you want...
The hard way:
If I wanted to step through various crystal values I would start with both a 20 and a 16 in circuit and feed them through an AND gate (to act as a switch) then into a 74HC161 to act as a divider and back through the AND gate to switch the divider output.
You could then get 20, 16, 10, 8, 5, 4, 2.5 and 2 and you could run your micro off the 20Mhz and generate anything you wanted below that using it's software.
The easy way:DS1077