clk: imx7d: Remove audio_mclk_root_clk

The audio_mclk_root_clk was added as a gate with the CCGR121 (0x4790),
but according to the reference manual, there is no such gate. The
CCGR121 belongs to ECSPI2 and it is not shared.

Fixes: 8f6d8094 ("ARM: imx: add imx7d clk tree support")
Reported-by: David Wolfe <david.wolfe@nxp.com>
Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20220127141052.1900174-2-abel.vesa@nxp.com