swiotlb: use depends on for DMA_RESTRICTED_POOL

Use depends on instead of select for DMA_RESTRICTED_POOL; otherwise it
will make SWIOTLB user configurable and cause compile errors for some
arch (e.g. mips).

Fixes: 0b84e4f8 ("swiotlb: Add restricted DMA pool initialization")
Acked-by: Will Deacon <will@kernel.org>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Claire Chang <tientzu@chromium.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>