pinctrl: stm32: handle a configuration list

This patch handles a configuration list behind pinctrl-0
like pinctrl-0 = <&qspi_clk_a &qspi_bk1_a &qspi_bk2_a>;

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Acked-by: Vikas MANOCHA <vikas.manocha@st.com>