armv8: include psci_update_dt() unconditionally

armv8: include psci_update_dt() unconditionally

psci_update_dt() is also required if CONFIG_ARMV8_PSCI is set, that is,
if u-boot is the PSCI provider.
Guard the check which is intended to call into the PSCI implementation
in the secure firmware, by the proper macro SEC_FIRMWARE_ARMV8_PSCI.

Mark the function as weak because - unfortunately - there is already
a stub of the same function in arch/arm/mach-rmobile/psci-r8a779a0.c
which does not the same as the common one.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>