powerpc: Don't use --- in kernel logs

When a kernel log containing --- at the start of a line is copied into
a patch message, 'git am' drops everything located after that ---.

Replace --- by ---- to avoid that.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/54a1f8d2c3fb5b95434039724c8c141052ae5cc0.1739346038.git.christophe.leroy@csgroup.eu