ARM: dts: keystone: Add PSC node

The Power Sleep Controller (PSC) module is responsible
for the power and clock management for each of the peripherals
present on the SoC. Represent this as a syscon node so that
multiple users can leverage it for various functionalities.

Signed-off-by: Suman Anna <s-anna@ti.com>
[afd@ti.com: add simple-mfd compatible]
Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>