firmware: ti_sci: Add support to get TISCI handle using of_phandle

TISCI has been updated to have support for Resource management(like
interrupts etc..). And there can be multiple device instances of a
resource type in a SoC. So every driver corresponding to a resource type
should get a TISCI handle so that it can make TISCI calls. And each
DT node corresponding to a device should exist under its corresponding
bus node as per the SoC architecture.

But existing apis in TISCI library assumes that all TISCI users are
child nodes of TISCI. Which is not true in the above case. So introduce
(devm_)ti_sci_get_by_phandle() apis that can be used by TISCI users
to get TISCI handle using of phandle property.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>