libretro: Implement retro_set_controller_port_device()

libretro: Implement retro_set_controller_port_device()

Add the 'default' value to controller type variables and set it as the
default value and implements the retro_set_controller_port_device()
function. When the variable's value is 'default', the value set via the
function is used, otherwise the value of the variable is used.

This allows the frontends to use retro_set_controller_port_device() or
the variables, pleasing both the users and frontends prefering the
standard API and the ones prefering the custom API.