timer: sti: use clk API to get timer clock rate

Retrieve clock rate through device tree. This mimics the behavior of
arm_global_timer in Linux.

Signed-off-by: Nicolas Heemeryck <nicolas.heemeryck@gmail.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Acked-by: Patrice Chotard <patrice.chotard@st.com>