ARM: dts: dra7: Fix clock data for gmac_gmii_ref_clk_div

ARM: dts: dra7: Fix clock data for gmac_gmii_ref_clk_div

This commit fixes the clock data inside the DRA7xx clocks device tree
structure for the gmac_gmii_ref_clk_div clock. This clock is actually
the GMAC_MAIN_CLK and has nothing to do with the register at address
0x4a0093d0. If CLKSEL_REF bit 24 inside of CM_GMAC_GMAC_CLKCTRL, is
set to 1 in order to use the GMAC_RMII_CLK instead of the
GMAC_RMII_HS_CLK, the kernel generates a clock divider warning:
    WARNING: CPU: 0 PID: 0 at drivers/clk/clk-divider.c:129 clk_divider_recalc_rate+0xa8/0xe0()
    gmac_gmii_ref_clk_div: Zero divisor and CLK_DIVIDER_ALLOW_ZERO not set

By properly configuring the gmac_gmii_ref_clk_div (GMAC_MAIN_CLK) to
have the parent of dpll_gmac_m2_ck always divided by 2 the warning is
resolved and the clock tree is fixed up.

Additionally, a new clock called rmii_50mhz_clk_mux is defined that
does utilize CM_GMAC_GMAC_CLKCTRL[24] CLKSEL_REF to configure the
source clock for the RMII_50MHZ_CLK.

Cc: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: J.D. Schroeder <jay.schroeder@garmin.com>
Reviewed-by: Trenton Andres <trenton.andres@garmin.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>