Generalised the SPI abstraction layer to also be able to handle interfaces

that don't provide the full flexibility of a bit-bashing interface.

Temporarily hacked rtl8139.c to use the new interface.

src/include/gpxe/spi_bit.h

@@ -0,0 +1,45 @@
+#ifndef _GPXE_SPI_BIT_H
+#define _GPXE_SPI_BIT_H
+
+/** @file
+ *
+ * SPI bit-bashing interface
+ *
+ */
+
+#include <gpxe/spi.h>
+
+/** A bit-bashing SPI bus */
+struct spi_bit_basher {
+	/** SPI bus */
+	struct spi_bus bus;
+	/** Bit-bashing interface */
+	struct bit_basher basher;
+};
+
+/** Bit indices used for SPI bit-bashing interface */
+enum {
+	/** Serial clock */
+	SPI_BIT_SCLK = 0,
+	/** Master Out Slave In */
+	SPI_BIT_MOSI,
+	/** Master In Slave Out */
+	SPI_BIT_MISO,
+	/** Slave 0 select */
+	SPI_BIT_SS0,
+};
+
+/**
+ * Determine bit index for a particular slave
+ *
+ * @v slave		Slave number
+ * @ret index		Bit index (i.e. SPI_BIT_SSN, where N=slave) 
+ */
+#define SPI_BIT_SS( slave ) ( SPI_BIT_SS0 + (slave) )
+
+/** Delay between SCLK transitions */
+#define SPI_BIT_UDELAY 1
+
+extern void init_spi_bit_basher ( struct spi_bit_basher *spibit );
+
+#endif /* _GPXE_SPI_BIT_H */