altos: Get stm32l pwm driver working

Fix the CCMR1_OC1PE and CCMR2_OC3PE values.
Disable clock when no PWM outputs are running.
Fix the apb1enr value for the timer.
Set ARR value to PWM_MAX - 1 -- ARR is off by one.
Sets the GPIO pins to 40MHz bandwidth for sharper edges.

Tested on EasyMega, but that code is not included as it breaks the
companion protocol.

Signed-off-by: Keith Packard <keithp@keithp.com>