drivers/spi/spi_flash: Use boot_device_spi_flash to obtain spi_flash structure
Instead of storing spi flash device structure in spi flash driver, use
boot_device_spi_flash callback to obtain pointer to boot device spi
flash structure.
BUG=b:
38330715
Change-Id: Idd50b7644d1a4be8b62d38cc9239feae2215103c
Signed-off-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: https://review.coreboot.org/19703
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>