staging: binder: Improve Kconfig entry for ANDROID_BINDER_IPC_32BIT

Add a more clear explanation of the option in the prompt, and
make the config depend on ANDROID_BINDER_IPC being selected.

Also sets the default to y, which matches AOSP.

Cc: Colin Cross <ccross@android.com>
Cc: Arve Hjønnevåg <arve@android.com>
Cc: Serban Constantinescu <serban.constantinescu@arm.com>
Cc: Android Kernel Team <kernel-team@android.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/android/Kconfig

@@ -21,8 +21,9 @@ config ANDROID_BINDER_IPC
 	  between said processes.
 
 config ANDROID_BINDER_IPC_32BIT
-	bool "Use old 32-bit binder api"
-	depends on !64BIT
+	bool "Use old (Android 4.4 and earlier) 32-bit binder API"
+	depends on !64BIT && ANDROID_BINDER_IPC
+	default y
 	---help---
 	  The Binder API has been changed to support both 32 and 64bit
 	  applications in a mixed environment.