mpc85xx/tlb.c: Allow platforms to specify wimge bits

Some platforms might want to override the default wimge=0 for
DDR.  Add CONFIG_SYS_PPC_DDR_WIMGE for those platforms to use.
This will initially only be used by TQM85xx, but could be
useful for other boards or testing going forward.  Note that
the name of this define is not 85xx-specific.  WIMGE is a
fairly universal concept, so any ppc platforms that require
different WIMGE settings for DDR can use the same #define.

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