mtd/cfi: add support for SST 4KB sector granularity

Add support for SST 4KB sector granularity.

Many recent SST flashes, i.e. SST39VF3201B and similar of this family
are declared CFI-conformant from SST. They support CFI query, but implement
2 different sector sizes in the same memory: a 64KB sector (they call it
"block", std AMD erase cmd=0x30), and a 4KB sector (they call it "sector",
erase cmd=0x50). Also, CFI query on these chips, reading from address 0x2dh
of cfi query struct, detects a number of secotrs for the 4KB granularity
(flinfo shows it).

For all other aspects, they are CFI compliant, so, as Linux do, i think
it's a good idea to handle these chips in the CFI driver, with a fixup
to allow 4KB granularity, as should be expected, instead of 64KB.

Signed-off-by: Angelo Dureghello <sysamfw@gmail.com>
Signed-off-by: Stefan Rose <sr@denx.de>

include/flash.h

@@ -44,6 +44,7 @@ typedef struct {
 	ulong	buffer_write_tout;	/* maximum buffer write timeout		*/
 	ushort	vendor;			/* the primary vendor id		*/
 	ushort	cmd_reset;		/* vendor specific reset command	*/
+	uchar   cmd_erase_sector;	/* vendor specific erase sect. command	*/
 	ushort	interface;		/* used for x8/x16 adjustments		*/
 	ushort	legacy_unlock;		/* support Intel legacy (un)locking	*/
 	ushort	manufacturer_id;	/* manufacturer id			*/