cros_ec: Use a hyphen in the uclass name

cros_ec: Use a hyphen in the uclass name

Device-tree rules require that aliases use a hyphen rather than a
underscore. Update the uclass name to fit with this.

This allows device-tree aliases to be used to refer to cros-ec devices,
for example:

	aliases {
	    cros-ec0 = &ec;
	    cros-ec1 = &pd;
	};

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>