spi: nxp-flexspi: Add i.MX platform dependency

This driver also supports various i.MX8 platforms. Add ARCH_MXC for
selecting this driver without Layerscape support.

Fixes: c6b15b24 ("spi: nxp-flexspi: Fix ARCH_LAYERSCAPE dependency")
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Link: https://lore.kernel.org/r/20230313083621.154729-1-alexander.stein@ew.tq-group.com


Signed-off-by: Mark Brown <broonie@kernel.org>