Backport few fixes.
    
    Backport "northd: Add ECMP support to router policies." (#1881826)
    Backport "Add missing documentation for router policy and ecmp sym reply stage." (#1881826)
    Backport "northd: add reject action for lb with no backends" (#1883957)
    Backport "northd: Fix iteration over vip backends." (#1904489)
    Backport "pinctrl: Honor always_learn_from_arp_request for self created MAC_Bindings." (#1903199)
    
    Resolves: #1881826
    Resolves: #1883957
    Resolves: #1904489
    Resolves: #1903199
    
    Signed-off-by: Numan Siddique <numans@ovn.org>