Preparations for Brushed PWM control

- move definitions for PULSE_1MS and timer frequencies to board.h
- split into input/output definitions

Brushed PWM needs faster timer for generating the output PWM
Support Brushed motors with raw PWM output

- Bump the PWM Output timers to 8 Mhz, enough for 5kHz PWM
- refine motor_pwm_rate from 50Hz to 5000Hz
  when higher then 1000Hz Brushed PWM will be selected
- Register output writer functions on PWM initialization
- pwmWriteBrushed() generates a 0-100% duty cycle PWM signal
  with motor_pwm_rate (1-5kHz) by values from 1000 to 2000
- mixer.c now only uses a generic pwmWriteOutput() function