ppc4xx_i2c: Implement directcntl register

ppc4xx_i2c: Implement directcntl register

As well as being able to generate its own i2c transactions, the ppc4xx
i2c controller has a DIRECTCNTL register which allows explicit control
of the i2c lines.

Using this register an OS can directly bitbang i2c operations. In
order to let emulated i2c devices respond to this, we need to wire up
the DIRECTCNTL register to qemu's bitbanged i2c handling code.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>