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Implement an I2C controller sharing mechanism between the host (kernel)
and PSP co-processor on some platforms equipped with AMD Cezanne SoC.

On these platforms we need to implement "software" i2c arbitration.
Default arbitration owner is PSP and kernel asks for acquire as well
as inform about release of the i2c bus via mailbox mechanism.

            +---------+
 <- ACQUIRE |         |
  +---------|   CPU   |\
  |         |         | \      +----------+  SDA
  |         +---------+  \     |          |-------
MAILBOX                   +--> |  I2C-DW  |  SCL
  |         +---------+        |          |-------
  |         |         |        +----------+
  +---------|   PSP   |
   <- ACK   |         |
            +---------+

            +---------+
 <- RELEASE |         |
  +---------|   CPU   |
  |         |         |        +----------+  SDA
  |         +---------+        |          |-------
MAILBOX                   +--> |  I2C-DW  |  SCL
  |         +--------...