platform/chrome: cros_ec_lpc: Merge cros_ec_lpc and cros_ec_lpc_reg

The cros_ec_lpc_reg files are only used by the cros_ec_lpc core and
there isn't logical separation between them. So, merge those files into
the cros_ec_lpc also allowing us to drop the header file used for the
interface between the two.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Nick Crews <ncrews@chromium.org>