buildroot

arch: add tune options for powerpc e5500 and e6500

These were forgotten when the subarches were added in cd88e49.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Alvaro Gamez <alvaro.gamez@hazent.com>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Tested-by: Alvaro G. M <alvaro.gamez@hazent.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>