serial: Add a SERIAL config

At present we have SPL_SERIAL and TPL_SERIAL but not piain SERIAL. This
works because there is a special build rule in Makefile that always
includes the drivers/serial directory.

It is better to have all driver directories included by drivers/Makefile
and there is already a rule in there for this purpose. It just needs a
Kconfig for U-Boot proper, so add one.

It is always enabled, for now, since that mimics the current behaviour.
It should be possible to drop the strange 'SERIAL_PRESENT' option at some
point and use SERIAL instead.

Signed-off-by: Simon Glass <sjg@chromium.org>