arm64: dts: imx8mp-dhcom-pdk3: Use symbolic macro for IOMUXC_SAI2_TXC__GPIO4_IO25

arm64: dts: imx8mp-dhcom-pdk3: Use symbolic macro for IOMUXC_SAI2_TXC__GPIO4_IO25

Currently, in order to configure IOMUXC_SAI2_TXC__GPIO4_IO25 a magic
raw value is written in this register. This makes the code not obvious
to read and modify.

Use the MX8MP_SION symbolic macro instead of the magic value to improve
code readability.

Signed-off-by: Eduard Bostina <egbostina@gmail.com>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>