Calculate the number of pages the GATT spans when reading from each page

Calculate the number of pages the GATT spans when reading from each page
to flush the TLB instead of hardcoding a size of 33 pages.  Apertures of
32MB and 64MB only use a 16 page GATT and an aperture of 128MB only uses
a 32 page GATT, so without this the code could walk off the end of the
pointer and cause a page fault if the next page was unmapped.  Also, for
aperture sizes > 128MB, not all of the pages would be read.  The Linux
driver has the same bug.

MFC after:	1 week
Tested by:	Frédéric PRACA  frederic.praca of freebsd-fr.org