arm: psci: add psci_cpu_can_disable() function

Some devices which support disable cpu0 can not disable cpu0,
so add psci_cpu_can_disable() function to make all cpus can
disable.

Change-Id: Iaed4129abfc1283c6371bc11bbe5ae4e66317280
Signed-off-by: Teng Fei Fan <tengfei@codeaurora.org>