drivers: Descend to drivers/soc unconditionally

Descend to drivers/soc directory unconditionally for SPL and U-Boot
builds. Individual drivers can have their own config to check what needs
to be built for SPL. There should be no increase in SPL code size
due to this change.

This is required on K3 SoCs to support DMA in SPL.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>