PCM5122 BQ-filter capability (6-band hardware equilizer)

[anabolyc]

This change implements

• PCM51XX driver changes similar to TAS58XX driver, allowing use of 6x internal BQ-filters. Unlike TAS58XX they are shared for both channels, have different coefficient format and different DSP memeory layout.
• PCM51xx driver implements change of process flow (5 built-in flows, where Process flow 5 actually allows use of BQ-filters)
• pcm51xx_settings module, similat to tas5805m_settings implementing NVS persistance for user settings
• changes in http_ui needed to implement UI part. UI reuses eq-settings page created for tas5805m before, with light modifications
  The final result looks like this

This is a draft, based on another feature branch for my convenience. I will clean up, rebase, and squash changes when #222 and #228 will be merged, but I appreciate early feedback