clk: stm32mp1: add MCO clocks

Two micro-controller clock output (MCO) pins are available:
MCO1 and MCO2.
For each output, it is possible to select a clock source.
The selected clock can be divided thanks to configurable
prescaler.

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>