samples: pmci: mctp: Add i2c+gpio endpoint sample

Adds sample showing i2c+gpio endpoint bindings in use.

Provides an overlay for the frdm-mcxn947 development board using JP8
pins for i2c and gpio connectivity.

The sample periodically sends a "ping" message to the bus controller and
prints out any messages it receives itself.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>