drivers/spi/spi_flash: Clean up SPI flash probe

1. Rename __spi_flash_probe to spi_flash_generic_probe and export it
so that drivers can use it outside spi_flash.c.
2. Make southbridge intel spi driver use spi_flash_generic_probe if
spi_is_multichip returns 0.
3. Add spi_flash_probe to spi_ctrlr structure to allow platforms to
provide specialized probe functions. With this change, the specialized
spi flash probe functions are now associated with a particular spi
ctrlr structure and no longer disconnected from the spi controller.

BUG=b:38330715

Change-Id: I35f3bd8ddc5e71515df3ef0c1c4b1a68ee56bf4b
Signed-off-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: https://review.coreboot.org/19708
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
8 files changed