clk: clk_stm32f: Move SYSCFG clock setup into configure_clocks()

Move SYSCFG clock setup into configure_clocks() instead of calling
clock_setup() from board file.

As this clock is only needed in case of ethernet enabled and as
both stm32f4 and stm32f7 are using the Designware ethernet IP,
we use CONFIG_ETH_DESIGNWARE to only enable this clock if needed.

Move the RMII setup from board_early_init_f() to board_init()
to insure that RMII bit is set only when clock driver is initialized.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>