metag: defconfigs: increase log buffer 8KiB => 128KiB

The Meta defconfigs set the log buffer size to just 8KiB, but with the
fairly recent conversion of the kernel log buffer into a structured
binary format, log messages appear to consume more space in the buffer,
and in some cases it's not big enough to store the entire boot log.

Therefore switch all the defconfigs to use the default size of 128KiB.

Signed-off-by: James Hogan <james.hogan@imgtec.com>