dts: nxp,mcux-i2s: Fix pinmux names after change

After a recent change for the imx gpr binding, the i2s driver was
changed to reflect the new names of the cells, but apparently also the
sai node can refer to itself with a phandle and need cells names to
match also.

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