uart: make poll_in and poll_out mandatory

Polling in/out are basic UART functions that are supposed to
work on every UART controllers. Therefore, both poll_in and
poll_out are now mandatory. The API no longer checks if
they are defined before calling. This is to avoid the if
statements so to speed up things a bit.

Change-Id: I3bd1e7a2dfa5405699ebaec3eb34cfc0434b0073
Signed-off-by: Daniel Leung <daniel.leung@intel.com>