drivers: base: move mutex lock out of add_memory_section()

There is no point in releasing the mutex for each section that is added
during boot time.  Just hold it over the entire initialization loop.

Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>