P2WI (Push/Pull 2 Wire Interface) is an SMBus like bus used to communicate
with some PMICs (like the AXP221).

Document P2WI DT bindings which are pretty much the same as the one defined
for the marvell's mv64xxx controller.
Signed-off-by: NBoris BREZILLON <boris.brezillon@free-electrons.com>
Signed-off-by: NWolfram Sang <wsa@the-dreams.de>