ARM: da850-evm: Remove SPI from SPL when booting from NAND

The da850evm boots from SPI flash while the da850evm_nand_defconfig
boots from NAND and these are configs are mutually exclusive.

Since there isn't a need for SPI during SPL in the NAND configuration,
this patch removes the SPI drivers in SPL while making them still
available in U-Boot proper.  This should save some code space in SPL.

Signed-off-by: Adam Ford <aford173@gmail.com>