serial/arc-uart: split probe from probe_earlyprintk

This is in preparation for devicetree based probing, where earlyprintk
won't have access to DT serial aliases which the normal probe would
absolutely rely on.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: linux-serial@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>