drivers: clock_control_mcux_syscon: Added get support for I2S FLEXCOMM

Previously, I2S_MCUX_FLEXCOMM was expected to use the external
MCLK only but it's possible for I2S_MCUX_FLEXCOMM to be sourced
from other clocks like the audio_pll_clk, in which case the
driver needs to be able to get the current clock frequency
in order to properly set dividers for requested sample rate.

Signed-off-by: Mike J. Chen <mjchen@google.com>