mpc85xx: switch default kernel to 5.15
The default kernel should be switched to 5.15 in order to enable testing
by a broader audience.
Tested on TP-Link TL-WDR4900 v1.
Acked-by: Josef Schlehofer <[email protected]>
Signed-off-by: Nick Hainke <[email protected]>
Nick Hainke