dm: serial: Allow the UART driver to be dropped from the image

In very very space-constrained devices even the full UART driver is too
large. In this case the debug UART can still be used in some cases.

Add options to enable the UART driver in SPL and U-Boot proper. Enable both
options by default.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Thomas Chou <thomas@wytron.com.tw>