arm64: dts: Fix various entry-method properties to reflect documentation

The idle-states binding documentation[1] mentions that the
'entry-method' property is required on 64-bit platforms and must be
set to "psci".

Linux commit a13f18f59d26 ("Documentation: arm: Fix typo in the idle-states
bindings examples") attempted to fix this earlier but clearly more is
needed.

Linux docs:
Documentation/devicetree/bindings/arm/idle-states.txt (see
idle-states node)

Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Li Yang <leoyang.li@nxp.com>
Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>