board/tqm85xx: Create and tear down TLB for get_ram_size()

We need a TLB entry to call get_ram_size(); the common code doesn't create
one until *after* fixed_sdram() has determined the size.  So we set up tlbs
for the max possible size and tear them down once we're done with
get_ram_size(); the common 85xx code will then set up a final set of tlb
entries for the *actual* detected size of ddr.

This prevents us from having TLB entries that are larger than DDR sitting
around for very long, which is not a recommended scenario.

Signed-off-by: Becky Bruce <beckyb@kernel.crashing.org>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>