spl: Kconfig: dfu: spl-dfu depends on SPL_RAM_SUPPORT

Since SPL_DFU_SUPPORT is depends on SPL_RAM_SUPPORT,
hence select SPL_DFU_SUPPORT only when
SPL_RAM_SUPPORT is chosen.

Signed-off-by: Ravi Babu <ravibabu@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>