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>