drivers: pwm: rpi_pico: Convert clk calculations to fixed-point

Remove the use of floating point to calculate in
`pwm_rpi_get_cycles_per_sec` to fixed point calculations that result in the
same value.  There is still a division here, which is somewhat slow still,
but the end result about twice fast for the whole path through the led pwm
code.

Even better would be to cache these values, since there is no interface in
this driver to change the configuration past init time.

Signed-off-by: David Brown <david.brown@linaro.org>