alif/machine_pwm: Implement machine.PWM.

This commit adds `machine.PWM` support to the alif port.  It uses the
existing common machine bindings and implements the standard set of
functionality: `freq()`, `duty_u16()`, `duty_ns()` and `invert`.

It uses the UTIMER peripheral and makes PWM available on all pins that have
an alt function connection to a UTIMER, which is 54 pins.  It does not use
UTIMER11 which is already in use by the HE core for its systick timer.  So
the following pins don't have PWM available because they need UTIMER11:
P2_6, P2_7, P7_6, P7_7, P12_6, P12_7.

Signed-off-by: Damien George <damien@micropython.org>

Author: dpgeorge