sf: Add a method to obtain the block-protect setting

It is useful to obtain the block-protect setting of the SPI flash, so we
know whether it is fully open or (perhaps partially) write-protected. Add
a method for this. Update the sandbox driver to process this operation and
add a test.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/mtd/spi/spi_flash.c b/drivers/mtd/spi/spi_flash.c
index a87bacd..0c2392f 100644
--- a/drivers/mtd/spi/spi_flash.c
+++ b/drivers/mtd/spi/spi_flash.c
@@ -110,6 +110,18 @@
 }
 #endif
 
+int spi_flash_cmd_get_sw_write_prot(struct spi_flash *flash)
+{
+	u8 status;
+	int ret;
+
+	ret = read_sr(flash, &status);
+	if (ret)
+		return ret;
+
+	return (status >> 2) & 7;
+}
+
 #ifdef CONFIG_SPI_FLASH_BAR
 /*
  * This "clean_bar" is necessary in a situation when one was accessing