clk: ti: dra7: fix "failed to lookup clock node gmac_gmii_ref_clk_div" boot message

Prevent creating clk alias for non existing gmac_gmii_ref_clk_div clock and,
this way, eliminate excessive error message during boot:

 "ti_dt_clocks_register: failed to lookup clock node gmac_gmii_ref_clk_div"

Fixes: c097338e

 ("ARM: dts: dra7: cpsw: fix clocks tree")
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>