drivers: pwm: silabs: Add LETIMER PWM driver for Series 2

Add PWM driver for the Low Energy Timer peripheral on Series 2.
The LETIMER runs at up to 32 kHz and has a 24-bit counter. It
only supports PWM output, it does not support input capture.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>