spi: imx: enable loopback only for ECSPI controller family

Limit SPI_LOOP mode to ECSPI controller (iMX.51, iMX53 and i.MX6) only since
there is no support in other families specific code for now.

Signed-off-by: Anton Bondarenko <anton.bondarenko.sama@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>