clk: ti: Implement FAPLL set_rate for the PLL

Since we have a fractional divider for the synthesizer, just implement
a simple multiply logic for the PLL.

It seems the PLL divider needs to have also the multiplier set for the PLL
to lock. At least I have not yet figured out if divided rates are doable.

So let's just ignore the PLL divider for now as the synthesizer has both
integer and fractional dividers so we don't even need to use the PLL
divider for the rates we know work with PLL locking.

Cc: Brian Hutchinson <b.hutchman@gmail.com>
Cc: Matthijs van Duin <matthijsvanduin@gmail.com>
Cc: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>