nfc: pn532: Add uart phy docs and rename it

This adds documentation about the uart phy to the pn532 binding doc. As
the filename "pn533-i2c.txt" is not appropriate any more, rename it to
the more general "pn532.txt".
This also documents the deprecation of the compatible strings ending
with "...-i2c".

Cc: Johan Hovold <johan@kernel.org>
Cc: Simon Horman <horms@verge.net.au>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lars Poeschel <poeschel@lemonage.de>
Signed-off-by: David S. Miller <davem@davemloft.net>