clk: stm32mp1: add Peripheral & Kernel Clocks

Each peripheral requires a bus interface clock.
Some peripherals need also a dedicated clock for their communication
interface, this clock is generally asynchronous with respect to the bus
interface clock (peripheral clock), and is named kernel clock.

For each IP, Peripheral clock and Kernel are generally gating with same
gate. Also, Kernel clocks can share a same multiplexer.
This patch introduces a mechanism to manage a gate with several
clocks and to manage a shared multiplexer (mgate and mmux).
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>