am33xx: config.mk: Fix option used to enable SPI SPL image type

The option SPL_SPI_SUPPORT is used to enable support in SPL for loading
images from SPI flash, it should not be used to determine the build type
of the SPL image itself. The ability to read images from SPI flash does
not imply the SPL will be booted from SPI flash.

Unconditionally build SPI flash compatible SPL images.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Acked-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>