doc: workaround for __deprecated functions

This edit to the doxygen.config file causes Doxygen to treat
__deprecated as a predefined macro, which expands to nothing.
See https://wiki.zephyrproject.org/view/Function_Documentation#Workarounds


for further information about this workaround.

Change-Id: I8e344cf65d7ff45609d5dd9a0caec14df9799d46
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>