doc: fix doxygen warnings 'documented symbol `XYZ` was not declared or defined.'

The problem is doxygen's parser is getting confused by constructs as:

  static inline __attribute__((always_inline))
 	void sys_out8(uint8_t data, io_port_t port)
  {
 	_arc_v2_aux_reg_write(port, data);
  }

Too many words at the beginning of the function definition. So change
to use the macro ALWAYS_INLINE (which is already defined to mean
'inline __attribute__((always_inline))`.

Kills:

sys_io.h:37: warning: documented symbol `static inline void sys_out8' was not declared or defined.
sys_io.h:47: warning: documented symbol `static inline uint8_t sys_in8' was not declared or defined.
sys_io.h:58: warning: documented symbol `static inline void sys_out16' was not declared or defined.
sys_io.h:68: warning: documented symbol `static inline uint16_t sys_in16' was not declared or defined.
sys_io.h:79: warning: documented symbol `static inline void sys_out32' was not declared or defined.
sys_io.h:89: warning: documented symbol `static inline uint32_t sys_in32' was not declared or defined.
sys_io.h:120: warning: documented symbol `static inline int sys_io_test_bit' was not declared or defined.
sys_io.h:133: warning: documented symbol `static inline int sys_io_test_and_set_bit' was not declared or defined.
sys_io.h:146: warning: documented symbol `static inline int sys_io_test_and_clear_bit' was not declared or defined.
sys_io.h:161: warning: documented symbol `static inline void sys_write8' was not declared or defined.
sys_io.h:171: warning: documented symbol `static inline uint8_t sys_read8' was not declared or defined.
sys_io.h:182: warning: documented symbol `static inline void sys_write16' was not declared or defined.
sys_io.h:192: warning: documented symbol `static inline uint16_t sys_read16' was not declared or defined.
sys_io.h:248: warning: documented symbol `static inline int sys_test_bit' was not declared or defined.
sys_io.h:261: warning: documented symbol `static inline int sys_test_and_set_bit' was not declared or defined.
sys_io.h:274: warning: documented symbol `static inline int sys_test_and_clear_bit' was not declared or defined.

Change-Id: Id10e9b6cd44a370ccc732c17b23fb66bd1845205
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>