pci: controller: msm: Remove support for loopback

Debugfs support for loopback is obsolete. This is
not supported on current or future platforms.
Remove it from debugfs to prevent any security issues
as it allows to access DDR.

Change-Id: I499588e9e28430c4358d2d8e2364430c1acab82e
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>