stm32mp1: clk: configure pll1 with OPP

The PLL1 node (st,pll1) is optional in device tree, the max supported
frequency define in OPP node is used when the node is absent.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>