NFC: Add basic NCI over SPI

The NFC Forum defines a transport interface based on
Serial Peripheral Interface (SPI) for the NFC Controller
Interface (NCI).

This module implements the SPI transport of NCI, calling SPI module
directly to read/write data to NFC controller (NFCC).

NFCC driver should provide functions performing device open and close.
It should also provide functions asserting/de-asserting interruption
to prevent TX/RX race conditions.
NFCC driver can also fix a delay between transactions if needed by
the hardware.

Signed-off-by: Frederic Danis <frederic.danis@linux.intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
diff --git a/net/nfc/nci/Kconfig b/net/nfc/nci/Kconfig
index 6d69b5f..2a24160 100644
--- a/net/nfc/nci/Kconfig
+++ b/net/nfc/nci/Kconfig
@@ -8,3 +8,13 @@
 
 	  Say Y here to compile NCI support into the kernel or say M to
 	  compile it as module (nci).
+
+config NFC_NCI_SPI
+	depends on NFC_NCI && SPI
+	bool "NCI over SPI protocol support"
+	default n
+	help
+	  NCI (NFC Controller Interface) is a communication protocol between
+	  an NFC Controller (NFCC) and a Device Host (DH).
+
+	  Say yes if you use an NCI driver that requires SPI link layer.