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>