SGTL5000's LINEOUT_R Used as Mono

The SGTL5000's right channel is used to drive the earpiece speaker, this was noticed after the prototypes were produced but was intentionally not changed in the final design so as to keep software compatibility between the prototypes and final units.

This implies that the software driver needs route all mono audio to the right channel, and keep the left channel muted.

The audio codec's MONOMODE_DAC and DAC_MUTE_LEFT should be set HIGH in order to use the right channel as a mono output. See its datasheet here: https://www.nxp.com/docs/en/data-sheet/SGTL5000.pdf