Add a pmap_kremove_device() to undo mappings made with pmap_kenter_device().

Add a pmap_kremove_device() to undo mappings made with pmap_kenter_device().

Previously we used pmap_kremove(), but with ARM_NEW_PMAP it does the remove
in a way that isn't SMP-coherent (which is appropriate in some circumstances
such as mapping/unmapping sf buffers).  With matching enter/remove routines
for device mappings, each low-level implementation can do the right thing.

Reviewed by:	Svatopluk Kraus <onwahe@gmail.com>