ASoC: SOF: Rename i.MX8 platform to i.MX8X

i.MX8 and i.MX8X platforms are very similar and were treated the same.
Anyhow, we need to account for the differences somehow.

Current supported platform is i.MX8QXP which is from i.MX8X family.
Rename i.MX8 platform to i.MX8X to prepare for future i.MX8 platforms.

Signed-off-by: Paul Olaru <paul.olaru@nxp.com>
Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200210095817.13226-2-daniel.baluta@oss.nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>