test: frequency and pure math functions extraction

[btpsj]

Several parts of the cxd56-hal have a pure math calculation (frequencies, etc.) that have been incorrectly calculated before and could be validated via tests. Extract the pure math components of the these kinds of functions and write tests for them. These include

• cxd56-hal/src/pwm.rs:95 compute_param_phase
• cxd56-hal/src/timer.rs:171 compute_load
• cxd56-hal/src/watchdog.rs:73 timeout_to_load
• cxd56-hal/src/spi.rs:650 baud_divisors