Set AUDIO_EXT_REFCLK1, used as I2S_1_MCLK on Verdin AM62 family, to 25MHz
(this is the only valid option according to TI [1]).

[1] https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1188051/am625-audio_ext_refclk1-clock-output---dts-support/4476322#4476322Reviewed-by: Jai Luthra <j-luthra@ti.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Link: https://lore.kernel.org/r/20230807202159.13095-3-francesco@dolcini.itSigned-off-by: Nishanth Menon <nm@ti.com>