i2c: designware: Add support for bus clear feature

Newer versions of the DesignWare I2C block support the detection of
stuck signals, and a mechanism to recover from them. Add the required
software support to the driver.

This change was prompted by the observation that reading a single byte
from register 0 of a VEML7700 seems to cause it to issue an ACK too
early, and the controller to complain about losing arbitration. There
is a suspicion that this may be a more widespread problem, but at least
this patch prevents the bus from locking up.

See: https://github.com/raspberrypi/linux/issues/6057



Signed-off-by: Phil Elwell <phil@raspberrypi.com>