platform/chrome: cros_ec_proto: Add response tracing

Add the ability to view response codes as well.

I dropped the EVENT_CLASS since there is only one event per class.

cros_ec_cmd has now been renamed to cros_ec_request_start.

Example:
$ echo 1 > /sys/kernel/debug/tracing/events/cros_ec/enable
$ cat /sys/kernel/debug/tracing/trace

369.416372: cros_ec_request_start: version: 0, command: EC_CMD_USB_PD_POWER_INFO
369.420528: cros_ec_request_done: version: 0, command: EC_CMD_USB_PD_POWER_INFO, ec result: EC_RES_SUCCESS, retval: 16
369.420529: cros_ec_request_start: version: 0, command: EC_CMD_USB_PD_DISCOVERY
369.421383: cros_ec_request_done: version: 0, command: EC_CMD_USB_PD_DISCOVERY, ec result: EC_RES_SUCCESS, retval: 5

Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>