clk: stm32f4: Add LSI & LSE clocks

This patch introduces the support of the LSI & LSE clocks.
The clock drivers needs to disable the power domain write protection
using syscon/regmap to enable these clocks.

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>