clk: imx8mp: Remove IPG_AUDIO_ROOT from imx8mp-clock.h

clk: imx8mp: Remove IPG_AUDIO_ROOT from imx8mp-clock.h

Since the commit b24e288d5063 ("clk: imx: Remove the audio ipg clock
from imx8mp") removes the non-existing IPG_AUDIO_ROOT from the
clk-imx8mp.c, and this definition is not used by anywhere, let us
removed it in the imx8mp-clock.h as well.

Signed-off-by: Hui Wang <hui.wang@canonical.com>
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Link: https://lore.kernel.org/r/20211109125657.63485-1-hui.wang@canonical.com
Signed-off-by: Abel Vesa <abel.vesa@nxp.com>