[rpi] Add support for the second PWM channel

Add support for the second channel to bcm2835_pwm driver. Configurable
parameters like mode, period, ratio are exposed as sysctls with postfix '2',
e.g.: dev.pwm.N.mode2, dev.pwm.N.period2, dev.pwm.N.ratio2

Second channel can be enabled in DTB by configuring pwn-2chan overlay
instead of pwm in config.txt. See [1]

[1] https://github.com/raspberrypi/firmware/blob/master/boot/overlays/README

Submitted by: Bob Frazier
Differential Revision: https://reviews.freebsd.org/D15769