arm: imx6: cm_fx6: Enable DM SPI and SPI_FLASH, fix SPL build errors

arm: imx6: cm_fx6: Enable DM SPI and SPI_FLASH, fix SPL build errors

Enable driver model for SPI and SPI_FLASH to remove the following
compile warning on CM-FX6 SOM:
===================== WARNING ======================
This board does not use CONFIG_DM_SPI_FLASH. Please update
the board to use CONFIG_SPI_FLASH before the v2019.07 release.
====================================================

This change introduced SPL build error as shown:

In file included from include/common.h:47:0,
                 from drivers/mtd/spi/sf_probe.c:10:
drivers/mtd/spi/sf_probe.c: In function 'spi_flash_std_probe':
drivers/mtd/spi/sf_probe.c:149:54: error: dereferencing pointer to incomplete type 'struct dm_spi_slave_platdata'
scripts/Makefile.build:278: recipe for target 'spl/drivers/mtd/spi/sf_probe.o' failed
make[3]: *** [spl/drivers/mtd/spi/sf_probe.o] Error 1
scripts/Makefile.build:432: recipe for target 'spl/drivers/mtd/spi' failed
make[2]: *** [spl/drivers/mtd/spi] Error 2

Disabling DM for SPI support(SPI and SF) in SPL resolves the issue.
Target was compile tested, build was clean.

Signed-off-by: Suniel Mahesh <sunil.m@techveda.org>