Enable evdev on ppc32

Enable evdev on ppc32 as well, similar to what was done i386 and amd64 in
r340387 and ppc64 in r340632.

Evdev can be used by X and is used by wayland to handle input devices.

Approved by: jhibbits
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D18049