docs: nfc: add it to the driver-api book

Most of the descriptions here are oriented to a Kernel developer.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

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+============================================================================
+Kernel driver for the NXP Semiconductors PN544 Near Field Communication chip
+============================================================================
+
+
+General
+-------
+
+The PN544 is an integrated transmission module for contactless
+communication. The driver goes under drives/nfc/ and is compiled as a
+module named "pn544".
+
+Host Interfaces: I2C, SPI and HSU, this driver supports currently only I2C.
+
+Protocols
+---------
+
+In the normal (HCI) mode and in the firmware update mode read and
+write functions behave a bit differently because the message formats
+or the protocols are different.
+
+In the normal (HCI) mode the protocol used is derived from the ETSI
+HCI specification. The firmware is updated using a specific protocol,
+which is different from HCI.
+
+HCI messages consist of an eight bit header and the message body. The
+header contains the message length. Maximum size for an HCI message is
+33. In HCI mode sent messages are tested for a correct
+checksum. Firmware update messages have the length in the second (MSB)
+and third (LSB) bytes of the message. The maximum FW message length is
+1024 bytes.
+
+For the ETSI HCI specification see
+http://www.etsi.org/WebSite/Technologies/ProtocolSpecification.aspx