adb: add property to disable direct reg 3 writes

adb: add property to disable direct reg 3 writes

MacOS 9 has a bug in its PMU driver whereby after configuring the ADB bus
devices it sends another write to reg 3 on both devices resetting them
both back to the same address.

Add a new disable_direct_reg3_writes property to ADBDevice to disable these
direct writes which can enabled just for the upcoming pmu-adb support.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>