fix(fvp): fdts: unify and fix PSCI nodes

fix(fvp): fdts: unify and fix PSCI nodes

The PSCI DT nodes used for the various fvp-base model variants provide
explicit function IDs, as required for the pre-v0.2 PSCI specification.
This prevents them from being used from both AArch32 and AArch64 DT
clients, and using this version of the PSCI spec is long deprecated
anyway.

Remove the old compatible string and the function properties, to
force clients to use the standard function IDs as described in the PSCI
spec. sys_poweroff and sys_reset were never standardised or used anyway.

There should be no client software around that cannot deal with PSCI
v0.2.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: Ie87deb9898eae79b7307c15bcefcd4b311d4dc22