arm: k3: Fix getting ti_sci handle

API get_ti_sci_handle() is relying on the device-tree node name
to be "dmsc" for probing the ti_sci device. But with the introduction
of debug messages for dmsc, the node name changed to dmsc@44083000.
Because of this ti_sci is never probed cause a boot failure. Instead
of relying on device-tree node name, use the first available firmware
node for probing ti_sci.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>