dm: clk: add missing .priv_auto_alloc_size() for stm32f7

Add missing .priv_auto_alloc_size() callback.
Previously private struct stm32_clk was not allocate
which leads to unpredictable behaviour

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>