common bmc mailbox driver

[Eugene.Cho at dell.com]

Hey Cyril,
   I was recently following your upstream submission attempt for the aspeed mbox/mailbox, was wondering if there was been any progress since?

Reason I ask - is Nuvoton also has a "mailbox" for bmc<->host communication, which does the same thing (shared memory + interrupts both ways), and was wondering if we could just have 1 driver for both, instead 2 separate drivers.

Glancing at the aspeed driver, only key difference I see is the setting and handling the interrupt. Thoughts on this?