clk: stm32mp1: add Kernel timers

This patch adds Kernel timers.
This patch adds timers kernel clock.
Timers are gather into two groups corresponding to the APB bus
they are attached to.
Each group has its own prescaler, managed in this patch.

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