cmd_ide: enhance new feature "CONFIG_IDE_AHB"

Although most IDE controller is designed to be connected to PCI bridge,
there are still some IDE controller support AHB interface for SoC design.

The driver implementation of these IDE-AHB controllers differ from other
IDE-PCI controller, some additional registers and commands access is required
during CMD/DATA I/O. Hence a configuration "CONFIG_IDE_AHB" in cmd_ide.c is
required to be defined to support these kinds of SoC controllers. Such as
Faraday's FTIDE020 series and Global Unichip's UINF-0301.

Signed-off-by: Macpaul Lin <macpaul@andestech.com>
diff --git a/README b/README
index d7e5b76..b74483e 100644
--- a/README
+++ b/README
@@ -2751,6 +2751,14 @@
 		source code. It is used to make hardware dependant
 		initializations.
 
+- CONFIG_IDE_AHB:
+		Most IDE controllers were designed to be connected with PCI
+		interface. Only few of them were designed for AHB interface.
+		When software is doing ATA command and data transfer to
+		IDE devices through IDE-AHB controller, some additional
+		registers accessing to these kind of IDE-AHB controller
+		is requierd.
+
 - CONFIG_SYS_IMMR:	Physical address of the Internal Memory.
 		DO NOT CHANGE unless you know exactly what you're
 		doing! (11-4) [MPC8xx/82xx systems only]