ARM: dts: stm32: Switch to MCO2 for PHY 50 MHz clock

The LAN8710i PHY currently uses 50 MHz clock direct from PLL4P.
To permit PLL4P to run at faster frequency, use MCO2 as a divider.
The PLL4P runs at 100 MHz, supplies MCO2 which divides it by 2 to
50MHz, and supplies the PHY with 50 MHz via pin PG2. The feedback
clock are fed back in via pin PA1.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrice Chotard <patrice.chotard@st.com>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Change-Id: Ia9bf7119785d49b633a3ae761c3dc4a30b92628a