serial/uart: adds new APIs for serial line control

This adds 2 new APIs to the serial/uart drivers:
() One is to control the serial line, such as RTS and CTS.
() Another one is to allow driver to expose hardware specific functions
   to apps. This is needed as some hardware may have extra registers to
   set for baud rate.

To keep the code size small, these features are disabled by default.

Change-Id: I15c000ce68a0a490dcfd3493b2fe9bc51fa974fa
Signed-off-by: Daniel Leung <daniel.leung@intel.com>