platsupport: Remove cmake warning for missing file

As it isn't an error for queried files to not exist, a
warning message is a bit misleading.

Signed-off-by: Kent McLeod <kent@kry10.com>

libplatsupport: Add tools to gen device headers

The new functions allow the user to input a YAML file that contains a
list of sub-devices of a particular device (GPIO pins, clocks on a
board etc), and output a C header '#define' enumeration.