spl: add RAM boot device only if it is actually defined

Some devices (e.g. dra7xx) support loading to RAM using DFU without
having direct boot from RAM support. Make sure the linker list
does not contain BOOT_DEVICE_RAM if CONFIG_SPL_RAM_DEVICE is not
enabled.

Fixes: 98136b2f ("spl: Convert spl_ram_load_image() to use linker list")

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>