ARM: dts: dra7: cpsw: fix clocks tree

Current clocks tree definition for CPSW/CPTS doesn't
correspond TRM for dra7/am57 SoCs.

CPTS: has to be sourced from gmac_rft_clk_mux clock
CPSW: DPLL_GMAC -> CLKOUT_M2 -> GMAC_250M_CLK -> 1/2 ->
      -> GMAC_MAIN_CLK (125 MHZ)

Hence, correct clock tree for GMAC_MAIN_CLK and use proper
clock for CPTS. This also require updating of CPTS clock
multiplier.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Mugunthan V N <mugunthanvnm@ti.com>
Acked-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>