ARM: STi: DT: STih407: STih410: Add clk_ignore_unused to kernel bootargs

At the moment we don't take a reference on some core interconnect
clocks which means when CCF turns off unused clocks the SoC will
hang. As a temp soltuion we will boot with clk_ignore_unused
parameter for all b2120 boards.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>