ec: Use DEVICE_NOOP macro formalism over static stub func

The in source comment:

	/* This function avoids an error on serial console. */

refers to the resource allocator needing to find a non-NULL
function pointer else complaints of "... missing read_resources"
will be spewed.

Unfortunately/fortunately (depending on the time of day) compiler
optimisers have gotten a bit better at optimising away no-op functions
leading to the very message these stubs attempted to avoid. By using
the DEVICE_NOOP formalism that is static inlined 'suggests' (not enforces)
to the compiler to keep these symbols around.

Change-Id: I182019627b6954a4020f9f70e9c829ce3135f63c
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/7598
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>