mtd: spi: Export spi_flash_std_probe()

With of-platdata we need to create drivers for particular chips, or at
least drivers that are separate from the standard code, since C structures
are created by dtoc which are private to that driver.

To avoid duplicating the probing code, export this probe function for use
by these drivers.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/drivers/mtd/spi/sf_probe.c b/drivers/mtd/spi/sf_probe.c
index f051e47..72b6ee7 100644
--- a/drivers/mtd/spi/sf_probe.c
+++ b/drivers/mtd/spi/sf_probe.c
@@ -137,7 +137,7 @@
 	return spi_flash_cmd_get_sw_write_prot(flash);
 }
 
-static int spi_flash_std_probe(struct udevice *dev)
+int spi_flash_std_probe(struct udevice *dev)
 {
 	struct spi_slave *slave = dev_get_parent_priv(dev);
 	struct dm_spi_slave_platdata *plat = dev_get_parent_platdata(dev);