doxygen: enable uart functions guarded by ifdef

Many of the UART interface functions are hidden behind ifdefs,
set those to 'y' when building documentation.

Change-Id: I1371e64b6fe4c269ce41c2c54e0f013d8088dbcf
Signed-off-by: Anas Nashif <anas.nashif@intel.com>
1 file changed