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/Kconfig b/drivers/ide/Kconfig
index 29dd99c..ec6b4a8 100644
--- a/drivers/ide/Kconfig
+++ b/drivers/ide/Kconfig
@@ -1035,8 +1035,8 @@
 	  Certain older chipsets, including the Tekram 690CD, use a single set
 	  of I/O ports at 0x1f0 to control up to four drives, instead of the
 	  customary two drives per port. Support for this can be enabled at
-	  runtime using the "ide0=four" kernel boot parameter if you say Y
-	  here.
+	  runtime using the "ide-4drives.probe" kernel boot parameter if you
+	  say Y here.
 
 config BLK_DEV_ALI14XX
 	tristate "ALI M14xx support"