clk: imx: add mux ops for i.MX8M composite clk

clk: imx: add mux ops for i.MX8M composite clk

The CORE/BUS root slice has following design, simplied graph:
The difference is core not have pre_div block.
A composite core/bus clk has 8 inputs for mux to select, saying clk[0-7].

It support target(smart) interface and normal interface. Target interface
is exported for programmer easy to configure ccm root. Normal interface
is also exported, but we not use it in our driver, because it will
introduce more complexity compared with target interface.

The normal interface simplified as below:
            SEL_A  GA
            +--+  +-+
            |  +->+ +------+
CLK[0-7]--->+  |  +-+      |
       |    |  |      +----v---+    +----+
       |    +--+      |pre_diva+---->    |  +---------+
       |              +--------+    |mux +--+post_div |
       |    +--+      |pre_divb+--->+    |  +---------+
       |    |  |      +----^---+    +----+
       +--->+  |  +-+      |
            |  +->+ +------+
            +--+  +-+
            SEL_B  GB

The mux in the upper pic is not the target interface MUX, target
interface MUX is hiding SEL_A and SEL_B. When you choose clk[0-7],
you are actually writing SEL_A or SEL_B depends on the internal
counter which will also control the internal "mux".

The target interface simplified as below which is used by Linux Kernel:
CLK[0-7]--->MUX-->Gate-->pre_div-->post_div

A requirement of the Target Interface's software is that the
target clock source is active, it means when setting SEL_A, the
current input clk to SEL_A must be active, same to SEL_B.

We touch target interface, but hardware logic actually also need
configure normal interface.

There will be system hang, when doing the following steps:
The initial state:
  SEL_A/SEL_B are both sourcing from clk0, the internal counter
  choose SEL_A.
1. switch mux from clk0 to clk1
   The hardware logic will choose SEL_B and configure SEL_B to clk1.
   SEL_A no changed.
2. gate off clk0
   Disable clk0, then the input to SEL_A is off.
3. swtich from clk1 to clk2
   The hardware logic will choose SEL_A and configure SEL_A to clk2,
   however the current SEL_A input clk0 is off, the system hang.

The solution to fix the issue is in step 1, write twice to
target interface MUX, it will make SEL_A/SEL_B both sources
from clk1, then no need to care about the state of clk0. And
finally system performs well.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>