arm64: psci: fix !CONFIG_HOTPLUG_CPU build warning

When building without CONFIG_HOTPLUG_CPU, GCC complains (rightly) that
psci_tos_resident_on is unused:

  arch/arm64/kernel/psci.c:61:13: warning: ‘psci_tos_resident_on’ defined but not used [-Wunused-function]
   static bool psci_tos_resident_on(int cpu)

As it's only ever used when CONFIG_HOTPLUG_CPU is selected, let's move
it into the existing ifdef.

Signed-off-by: Will Deacon <will.deacon@arm.com>
[Mark: write commit message]
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>