i2c: Don't try to unwedge unpowered busses.

If an i2c bus is known to be unpowered, we should not spend time trying
to unwedge it.  It's futile, so stop trying.  This commit adds a config
option,

  CONFIG_I2C_BUS_MAY_BE_UNPOWERED

which can be defined by a board if a bus may be unpowered during
runtime.

BUG=b:111683988
BRANCH=nocturne
TEST=Verify that unwedge attempts are skipped if the bus is deemed
unpowered.

Change-Id: Ice12b3957121be476ef0173a86f239f183010b47
Signed-off-by: Aseda Aboagye <aaboagye@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1182877
Commit-Ready: Aseda Aboagye <aaboagye@chromium.org>
Tested-by: Aseda Aboagye <aaboagye@chromium.org>
Reviewed-by: Alexandru M Stan <amstan@chromium.org>