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/southbridge/intel/fsp_rangeley/spi.c

@@ -719,9 +719,14 @@ spi_xfer_exit:
 	return 0;
 }
 
+static int xfer_vectors(const struct spi_slave *slave,
+			struct spi_op vectors[], size_t count)
+{
+	return spi_flash_vector_helper(slave, vectors, count, spi_ctrlr_xfer);
+}
+
 static const struct spi_ctrlr spi_ctrlr = {
-	.xfer = spi_ctrlr_xfer,
-	.xfer_vector = spi_xfer_two_vectors,
+	.xfer_vector = xfer_vectors,
 	.max_xfer_size = member_size(ich10_spi_regs, fdata),
 };