Arch does not include and dev rules for Raspberry Pi modules so by default librpip can use nothing. Librpip includes udev rules that can be used as is or modified to suit your requirement . From the source directory copy the ./distro/arch/librpip.rules to /etc/udev/rules.d
All Access Groups referred to in the udev.rules need to be created, e.g.
$ sudo groupadd gpio $ sudo groupadd spi $ sudo groupadd i2c
Additionally add the PWM group as used by the librpip-pwm-init script
$ sudo groupadd pwm
Now add the users that require access to the peripherals into the groups by editing /etc/group
PWM init script
Arch uses systemd to launch the librpip-pwm-init script instead of rc.local. From the source directory copy the ./distro/arch/pwm-init.service to /etc/systemd/system
This is enabled later when PWMs are actually configured.