ARM: keystone: dts: add psci command definition

This commit adds definition for cpu_on, cpu_off and cpu_suspend commands.
These definitions must match the corresponding PSCI definitions in
boot monitor.

Having those command and corresponding PSCI support in boot monitor allows
run time CPU hot plugin.

Signed-off-by: Vitaly Andrianov <vitalya@ti.com>
Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>