drivers/spi/spi-generic: Make spi_setup_slave strong symbol

Now that all platforms are updated to provide spi bus map, there is no
need to keep the spi_setup_slave as a weak symbol.

BUG=b:38430839

Change-Id: I59b9bbb5303dad7ce062958a0ab8dee49a4ec1e0
Signed-off-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: https://review.coreboot.org/19781
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/drivers/spi/spi-generic.c

@@ -112,11 +112,7 @@ void __attribute__((weak)) spi_init(void)
 	/* Default weak implementation - do nothing. */
 }
 
-const struct spi_ctrlr_buses spi_ctrlr_bus_map[0] __attribute__((weak));
-const size_t spi_ctrlr_bus_map_count __attribute__((weak));
-
-int __attribute__((weak)) spi_setup_slave(unsigned int bus, unsigned int cs,
-					  struct spi_slave *slave)
+int spi_setup_slave(unsigned int bus, unsigned int cs, struct spi_slave *slave)
 {
 	size_t i;