dm: serial: Reset the watchdog while waiting in getc()

dm: serial: Reset the watchdog while waiting in getc()

We have moved the busy-wait loop out of drivers and into the uclass. This
means that we must reset the watchdog when busy-waiting.

Note: some drivers may still have a busy-wait even with driver model, as
a transition mechanism. Driver model will tolerate this, and is can be
cleaned up when all users of the driver use driver model. An example is
ns16550.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@ti.com>