clk: stm32mp1: fix HSI divider flag

The divider of HSI (clk-hsi-div) is power of two divider.

Fixes: 9bee94e7 ("clk: stm32mp1: Introduce STM32MP1 clock driver")
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>