spl: Convert spl_spi_load_image() to use linker list

Add a linker list declaration for this method and remove the explicit
switch() code. Also set up the sunxi function.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/common/spl/spl_spi.c b/common/spl/spl_spi.c
index e4cc0d0..b9294f2 100644
--- a/common/spl/spl_spi.c
+++ b/common/spl/spl_spi.c
@@ -65,7 +65,7 @@
  * configured and available since this code loads the main U-Boot image
  * from SPI into SDRAM and starts it from there.
  */
-int spl_spi_load_image(struct spl_boot_device *bootdev)
+static int spl_spi_load_image(struct spl_boot_device *bootdev)
 {
 	int err = 0;
 	struct spi_flash *flash;
@@ -121,3 +121,5 @@
 
 	return err;
 }
+/* Use priorty 1 so that boards can override this */
+SPL_LOAD_IMAGE_METHOD(1, BOOT_DEVICE_SPI, spl_spi_load_image);