mpc83xx: use "A" nomenclature only on mpc834x and mpc836x families

marketing didn't extend their postpend-with-an-A naming strategy
on rev.2's and higher beyond the first two 83xx families.  This
patch stops us from misreporting we're running e.g., on an MPC8313EA,
when such a name doesn't exist.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>