drivers/spi/spi_flash: assume spi_flash read callback exists

spi_flash_erase() and spi_flash_write() already assume their
respective callbacks are supplied in the spi_flash_ops object.
Make the same assumption in spi_flash_read(). In order to do this
the spi_flash_ops objects from the drivers need to reference the
the previously used fallback read command, spi_flash_read_chunked().
This function is made global and renamed to spi_flash_cmd_read() for
consistency.

By doing this further dead code elimination can be achieved when the
spi flash drivers aren't included in the build.

A Hatch Chrome OS build achieves a further text segment reduction of
0.5KiB in verstage, romstage, and ramstage.

Change-Id: I7fee55e6ffc1983657c3adde025a0e8c9d12ca23
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38366
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/drivers/spi/atmel.c

@@ -104,6 +104,7 @@ static const struct atmel_spi_flash_params atmel_spi_flash_table[] = {
 };
 
 static const struct spi_flash_ops spi_flash_ops = {
+	.read = spi_flash_cmd_read,
 	.write = spi_flash_cmd_write_page_program,
 	.erase = spi_flash_cmd_erase,
 };