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/spi_flash_internal.h

@@ -66,6 +66,9 @@ int spi_flash_cmd_status(const struct spi_flash *flash, u8 *reg);
 int spi_flash_cmd_write_page_program(const struct spi_flash *flash, u32 offset,
 				size_t len, const void *buf);
 
+/* Read len bytes into buf at offset. */
+int spi_flash_cmd_read(const struct spi_flash *flash, u32 offset, size_t len, void *buf);
+
 /* Manufacturer-specific probe functions */
 int spi_flash_probe_spansion(const struct spi_slave *spi, u8 *idcode,
 			     struct spi_flash *flash);