serial: earlycon: Refactor earlycon registration

Separate earlycon matching from registration; add register_earlycon
which initializes and registers the matched earlycon.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>