clk: clk_stm32: Add .set_rate callback

Since 'commit f4fcba5c ("clk: implement clk_set_defaults()")'
STM32F4 family board can't boot.

Above patch calls clk_set_rate() for all nodes with assigned-clock-rates
property. Clock driver for STM32F family doesn't implement .set_rate
callback which make clk_set_defaults() exit on error and prevent board
to boot.

Fixes: f4fcba5c ("clk: implement clk_set_defaults()")
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>