sstate: set mode explicitly when creating directories in sstate-cache

When creating directories in the sstate-cache, explicitly set the mode
passed to mkdir to 0775 so that the directories are group writable, as
otherwise they cannot be shared with other users.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>