fix(fvp): fdts: Fix idle-states entry method

When firmware implements idle states via PSCI, the value of the DT
entry-method property must be "psci", not "arm,psci".

Fix this to make the CPU description binding compliant.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: Icd1bf704d177368af9b7aab545f47e580791b8cc
1 file changed