dt-bindings: Document STM32MP1 Reset Clock Controller (RCC) bindings

The RCC block is responsible of the management of the clock and reset
generation for the complete circuit.

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>