hal: Update to new serial API

This version of the HAL includes updates to the serial API, so update
our use of the HAL Serial API at the same time.

The new serial API doesn't provide a generic initialization function. As
such, we add a new platform-specific "give me a serial instance"
function, get_example_serial(), for use by examples needing a single
serial port. This allows examples to be portable across targets that
have an implementation of get_example_serial(). Only the
mdh-arm-an552-mps3 target provides an implementation of
get_example_serial() so far.

Signed-off-by:  <jaeden.amero@arm.com>