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/sf_internal.h b/drivers/mtd/spi/sf_internal.h
index 26f5c7c..46a5044 100644
--- a/drivers/mtd/spi/sf_internal.h
+++ b/drivers/mtd/spi/sf_internal.h
@@ -170,6 +170,9 @@
 /* Flash erase(sectors) operation, support all possible erase commands */
 int spi_flash_cmd_erase_ops(struct spi_flash *flash, u32 offset, size_t len);
 
+/* Get software write-protect value (BP bits) */
+int spi_flash_cmd_get_sw_write_prot(struct spi_flash *flash);
+
 /* Lock stmicro spi flash region */
 int stm_lock(struct spi_flash *flash, u32 ofs, size_t len);