arm: dts: k3-am642-sk: Add sysreset controller node

The AM64x SoC uses a central Device Management and Security Controller
(DMSC) processor that manages all the low-level device controls
including the system-wide SoC reset. The system-wide reset is managed
through the system reset driver.

Add a sysreset controller node as a child of the dmsc node to enable
the "reset" command from U-Boot prompt for the K3 AM642 SK.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>