drivers/spi/spi_flash: Add page_size to struct spi_flash

Add a new member page_size to spi_flash structure so that the various
spi flash drivers can store this info in spi_flash along with the
other sizes (sector size and total size) during flash probe. This
removes the need to have {driver}_spi_flash structure in every spi
flash driver.

This is part of patch series to clean up the SPI flash and SPI driver
interface.

BUG=b:38330715

Change-Id: I0f83e52cb1041432b0b575a8ee3bd173cc038d1f
Signed-off-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: https://review.coreboot.org/19704
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/include/spi_flash.h

@@ -29,6 +29,7 @@ struct spi_flash {
 	const char *name;
 	u32 size;
 	u32 sector_size;
+	u32 page_size;
 	u8 erase_cmd;
 	u8 status_cmd;
 	/*