ARM: 8919/1: make unexported functions static

The psci_cpu_{disable,die,kill} functions are not
exported, so make them static to avoid the following
warnings:

arch/arm/kernel/psci_smp.c:54:5: warning: symbol 'psci_cpu_disable' was not declared. Should it be static?
arch/arm/kernel/psci_smp.c:67:6: warning: symbol 'psci_cpu_die' was not declared. Should it be static?
arch/arm/kernel/psci_smp.c:79:5: warning: symbol 'psci_cpu_kill' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>