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

Add PWM driver for the Timer peripheral on Series 2.
The TIMER uses the high-frequency EM01 Group A clock, and has
a 16- or 32-bit counter. It supports PWM period and pulse capture
on channel 0, and PWM output on all channels.

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