i2c: aspeed: Add multi-master use case support

In multi-master environment, this driver's master cannot know
exactly when a peer master sends data to this driver's slave so
cases can be happened that this master tries sending data through
the master_xfer function but slave data from a peer master is still
being processed or slave xfer is started by a peer immediately
after it queues a master command. To support multi-master use cases
properly, this H/W provides arbitration in physical level and it
provides priority based command handling too to avoid conflicts in
multi-master environment, means that if a master and a slave events
happen at the same time, H/W will handle a higher priority event
first and a pending event will be handled when bus comes back to
the idle state.

To support this H/W feature properly, this patch adds the 'pending'
state of master and its handling code so that the pending master
xfer can be continued after slave operation properly.

Signed-off-by: Jae Hyun Yoo <jae.hyun.yoo@linux.intel.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>