dts: i2s_mcux_sai: Clarify IOMUXC GPR binding

Improve the documentation of the IOMUXC GPR binding so that it is
clear what the cells in the specifier space of "pinmux" space mean. And
change the names of the cells to be more appropriate. "offset" instead
of "pin" and "mask" instead of "function" describes more precisely what
the cells in the specifier mean.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>