ARM: OMAP: AM33XX: PM: Add support for VTT Toggle

In order to keep the DDR in Self-Refresh, CKE pin needs to be Pulled
down.
The VTT regulator Pull-up is on the same CKE pin. The pull down enabled
on CKE is a weak pull down.  This weak pull-down is not enough to pull
the line below 0.75v.
Because of this, the DDR never properly goes to self refresh and hence
low power state is not achieved.

This issue is addressed as below on a few boards
1. Add an external pull down on CKE pin and
2. Provide a method to disable VTT regulator (via gpio toggling)

The 1st part of the solution is addressed in h/w.
This patch set handles the 2nd part of toggling the gpio in the
suspend/resume path (depending on whether this toggling is requested
from DT)

Note:
This VTT regulator should be disabled after enabling self-refresh on
suspend, and should be enabled before disabling self-refresh on resume.
This is to allow proper self refresh entry/exit commands to be
transmitted to the memory (VTT needs to be enabled whenever you perform
any addr/ctrl signaling)

Here it is assumed that VTT Toggle will be done using a pin on GPIO-0
Instance.

Signed-off-by: Hebbar Gururaja <gururaja.hebbar@ti.com>
ARM: dts: AM33XX: EVM-SK: Add support for VTT Toggle

Due to a Hardware related issue, Suspend-Resume causes a hangs on
EVM-SK board.

On EVM-SK, the CKE signal does not go low properly during DDR self
refresh. The pull down enabled on CKE is a weak pull down, which is not
enough to overcome the VTT pull up to 0.75V.  Thus, the DDR never
properly goes to self refresh. CKE needs to be pulled down to keep the
DDR in Self refresh.

EVM-SK (Rev 1.2) has an added external pull down on CKE and the ability
to disable VTT regulator via gpio to address the above issues.

AM335x PM code provides an option to toggle VTT which can be enabled
through DT.

Add necessary DT nodes to toggle VTT on EVM-SK board.
Also, add the required pin-mux setup for the gpio pins.

Signed-off-by: Hebbar Gururaja <gururaja.hebbar@ti.com>