ide: add ide-4drives host driver (take 3)

CONFIG_BLK_DEV_4DRIVES deserves its own host driver:

* Add drivers/ide/legacy/ide-4drives.c and move "4drives" support there.

* Add ide-4drives.o in the link order after all other legacy host
  drivers enabled by "ide0=" options (they all are mutually exclusive).

* Make ide-4drives host driver probe itself for IDE devices instead of
  indirectly depending on ide_generic host driver.

* Add "probe" module parameter to ide-4drives and update documentation.

v2:
* s/paramater/parameter/ in ide.txt. (Noticed by Randy Dunlap)

v3:
* s/ide_4drives.probe/ide-4drives.probe/ in help entry.
  (Noticed by Sergei Shtylyov)

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
diff --git a/drivers/ide/legacy/Makefile b/drivers/ide/legacy/Makefile
index 7043ec7..6939329 100644
--- a/drivers/ide/legacy/Makefile
+++ b/drivers/ide/legacy/Makefile
@@ -6,6 +6,7 @@
 obj-$(CONFIG_BLK_DEV_DTC2278)		+= dtc2278.o
 obj-$(CONFIG_BLK_DEV_HT6560B)		+= ht6560b.o
 obj-$(CONFIG_BLK_DEV_QD65XX)		+= qd65xx.o
+obj-$(CONFIG_BLK_DEV_4DRIVES)		+= ide-4drives.o
 
 obj-$(CONFIG_BLK_DEV_GAYLE)		+= gayle.o
 obj-$(CONFIG_BLK_DEV_FALCON_IDE)	+= falconide.o