Currently at a fixed frequency of 1 kHz and with a fixed duty
cycle of 10%, but PWM does work.
As it turns out, PIO0_11 is not usable for PWM, because its timer
is already in use for the Step timer, and had to be disabled for
Gen7-ARM.
Test: define a heater in board.gen7-arm.h and a square signal
of 1 kHz with 10% duty cycle should appear on the heater pin.
Markus Hitter