mpc85xx: use generic diag.sh

mpc85xx: use generic diag.sh

This commit removes the target-specific diag.sh script. This way, the
generic one is used for the target, which uses DT-aliases to specify the
LEDs used.

This way, we are also able to use different LEDs to indicate different
states. We use green status LEDs for indicating boot and a running
system. Where possible, the red status LED is used to indicate failsafe
mode and a running upgrade.

Signed-off-by: David Bauer <mail@david-bauer.net>