Use available DM clk_stm32f.c driver instead of dedicated
mach-stm32/stm32f4/clock.c.
Migrate periph_clock defines from stm32_periph.h directly in
CLK driver. These periph_clock defines will be removed when STMMAC,
TIMER2 and SYSCFG drivers will support DM CLK.
Enable also CLK flag.
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>