platform/chrome: Centralize cros_ec_device allocation

platform/chrome: Centralize cros_ec_device allocation

Introduce a helper function, cros_ec_device_alloc(), to centralize the
allocation of the struct cros_ec_device.  Convert all protocol device
drivers to use this new function.

This is a preparatory step for separating common initialization logic
out of device drivers' probe() and cros_ec_register().

Link: https://lore.kernel.org/r/20250828083601.856083-2-tzungbi@kernel.org
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>