msm: ep_pcie: Add debugfs option to set the link speed

Add a debugfs option in the EP PCIE driver to set the link speed
to a user-specified value. Link must be retrained for the new setting
to take effect. Also add a log to print the link status register.

Change-Id: Ie4d9069d83fc8978208b03f792cb0e8ba665d395
Signed-off-by: Siva Kumar Akkireddi <sivaa@codeaurora.org>