powerpc: Fix bus_space_unmap

Previously it failed to compile since the macro passed too many
arguments to the function. Fix by adding the bus handle to the
function and adding an implementation that calls pmap_unmapdev.

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D43440

(cherry picked from commit a3d6e0de1c89c145b40ed80fe54c6bad1abbf416)