platform/chrome: cros_usbpd_notify: Move ec_command()

cros_ec_command() can be used by other modules too. So, move it to a
common location and export it.

This patch does not introduce any functional changes.

Signed-off-by: Prashant Malani <pmalani@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Link: https://lore.kernel.org/r/20210930022403.3358070-3-pmalani@chromium.org