device-pine64-pinephone: add kxkbrc configuration file for plasma-mobile to enable symbol row on pinephone-keyboard accessory

The default configuration of Plasma Mobile lacked the keyboard option lv3:ralt_switch and there is no GUI alternative to enable it at the moment. Consequently the PinePhone hardware keyboard could not produce common symbols such as +, -, _, =, \ because it relied on a Level3 layout switch. Using setxkbmap does not work due to Wayland. Adding the file /etc/xdg/kxkbrc allows such default configuration when installing pmOS. Can also be overriden in ~/.config/xkbrc. Fixes #1639 (closed)