logging: backend_adsp: lock the output

Adds a lock around the output to prevent corruption, and sets the buffer
size of the output to size of the output buffer. This fixes the
corrupted log output as process() may be called from multiple contexts
from different CPUs. A background log processing thread may race against
the panic ISR log context also calling process() on an SMP system.

Additionally sets the buffer size to 80 such that the cavs trace_out.c
functionality and lock are useful for more than one character at a time
greatly reducing the chances of garbled output in a printk and log race
but does not entirely prevent it. CONFIG_LOG_PRINTK=y should be used
to avoid all races.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>