uboot-imx

MLK-20886-1 misc: MU: Add MU driver to communicate with M4

Add a common iMX MU driver in misc uclass to communicate with M4.
The MU message format is defined to use 4 words as below, the driver
will use all 4 TR/RR in MU to pass one message

|WORD 0     | WORD 1      | WORD 2          | WORD 3         |
|SEQ        | TYPE        | PAYLOAD ADDRESS | PAYLOAD LENGTH |

 - SEQ:
   A sequence id starts from 0 and increases for each request message

 - TYPE:
   0x1:  Request.  Message sent from AP will set to this value.
   0x2:  Response. Message responded from M4 set to this value.
   0x3:  MU A side is ready.
   0x4:  MU B side is ready.

 - PAYLOAD ADDRESS:
   A pointer to the memory address where the uplayer message is stored

 - PAYLOAD LENGTH:
   The uplayer message length

Signed-off-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit aba0e51cc397e1d98be950f9c15619de06ebf782)