drivers/spi: reduce confusion in the API

Julius brought up confusion about the current spi api in [1]. In order
alleviate the confusion stemming from supporting x86 spi flash
controllers:

- Remove spi_xfer_two_vectors() which was fusing transactions to
  accomodate the limitations of the spi controllers themselves.
- Add spi_flash_vector_helper() for the x86 spi flash controllers to
  utilize in validating driver/controller current assumptions.
- Remove the xfer() callback in the x86 spi flash drivers which
  will trigger an error as these controllers can't support the api.

[1] https://mail.coreboot.org/pipermail/coreboot/2018-April/086561.html

Change-Id: Id88adc6ad5234c29a739d43521c5f344bb7d3217
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/25745
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/include/spi_flash.h

@@ -125,4 +125,18 @@ const struct spi_flash *boot_device_spi_flash(void);
 int spi_flash_ctrlr_protect_region(const struct spi_flash *flash,
 					const struct region *region);
 
+/*
+ * This function is provided to support spi flash command-response transactions.
+ * Only 2 vectors are supported and the 'func' is called with appropriate
+ * write and read buffers together. This can be used for chipsets that
+ * have specific spi flash controllers that don't conform to the normal
+ * spi xfer API because they are specialized controllers and not generic.
+ *
+ * Returns 0 on success and non-zero on failure.
+ */
+int spi_flash_vector_helper(const struct spi_slave *slave,
+	struct spi_op vectors[], size_t count,
+	int (*func)(const struct spi_slave *slave, const void *dout,
+		    size_t bytesout, void *din, size_t bytesin));
+
 #endif /* _SPI_FLASH_H_ */