Modify cpu nodes to set the enable-method

When using PSCI, the enable-method property for each CPU node in the
DTB needs to be set to "psci". Add a script and Makefile.am rules so
that this is done automatically for the final pass through DTC.

Signed-off-by: Matthew Leach <matthew.leach@arm.com>
[Mark: remove backticks, fix copyright date, use $(), fix indentation]
Signed-off-by: Mark Rutland <mark.rutland@arm.com>