drivers/spi/stmicro: Add '.op_erase' for N25Q256

'.op_erase' was not specified for this chip. Set it to sub sector
erase(CMD_M25PXX_SSE). Adjust page/sector size for sub sector erase
to work.

TEST=Untested, due to lack of hardware.

Change-Id: Icc2748fbd3afeb56693e1c17d97eb490fba67064
Signed-off-by: David Imhoff <dimhoff_devel@xs4all.nl>
Reviewed-on: http://review.coreboot.org/10077
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/drivers/spi/stmicro.c

@@ -165,9 +165,10 @@ static const struct stmicro_spi_flash_params stmicro_spi_flash_table[] = {
 	},
 	{
 		.device_id = STM_ID_N25Q256A,
+		.op_erase = CMD_M25PXX_SSE,
 		.page_size = 256,
-		.pages_per_sector = 256,
-		.nr_sectors = 512,
+		.pages_per_sector = 16,
+		.nr_sectors = 8192,
 		.name = "N25Q256A",
 	},
 };