spi/imx: Drop default setting for SPI_IMX

SPI_IMX is selected by imx_v6_v7_defconfig/imx_v4_v5_defconfig and we don't need
to have a default setting which depends on the IMX_HAVE_PLATFORM_SPI_IMX symbol.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>