[Non-BE] Resolve "Issues with EVS compatible stereo to mono downmix (quality on specific items + use of double)" - follow-up

Related to issue #947 (closed) and #567 (closed)

Reason why this change is needed

To address remarks on complexity for MR !1349 (merged) two minor extra fixes are suggested

Description of the change

• reduction of ROM table usage (BE) for POC

See code sections related to:

#ifdef NONBE_FIX_947_STEREO_DMX_EVS_POC
#define NONBE_FIX_947_STEREO_DMX_ROMOPT                       /* Orange: ROM optimisation for POC*/  
#endif

• reduction of complexity for fading in PHA

See code sections related to:

#ifdef NONBE_FIX_947_STEREO_DMX_EVS_PHA
#define NONBE_FIX_947_STEREO_DMX_FADOPT                       /* Orange: Fading optimisation */  
#endif   

• use of single precision for inv_sqrt commented as //#define NONBE_FIX_567_DOUBLE_STEREO_DM (waiting for final resolution on WMC tool udpate)

Affected operating points

• EVS stereo dmx
• complexity/storage is reduced (see details on page for issue #947 (closed))