clk: stm32mp1: fix mcu divider table

index 8: ck_mcu is divided by 256 (not 512)

Fixes: e51d297e ("clk: stm32mp1: add Sub System clocks")
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>