KVM: ARM: Change psci_call return value to bool

To avoid either using -1 or -EINVAL in return from kvm_psci_call change
the function to return a bool like the coprocessor handling functions
do.  Also clearly document this on on the function.

Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Christoffer Dall <c.dall@virtualopensystems.com>