conf: Fix XML parser for timer frequency
    
    The frequency is documented and formatted as an attribute of the <timer>
    element rather than a nested <frequency> element expected by the parser.
    Luckily enough, timer frequency has not been used by any driver so far.
    And users were not able to set it in the XML either.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>