asm-generic: Make dma-contiguous.h a mandatory include/asm header

asm-generic: Make dma-contiguous.h a mandatory include/asm header

dma-continuguous.h is generic for all architectures except arm32 which has
its own version.

Similar change was done for msi.h by commit 832071e2c970
("asm-generic: Make msi.h a mandatory include/asm header")

Suggested-by: Christoph Hellwig <hch@infradead.org>
Link: https://lore.kernel.org/linux-arm-kernel/20200117080446.GA8980@lst.de/T/#m92bb56b04161057635d4142e1b3b9b6b0a70122e
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Paul Walmsley <paul.walmsley@sifive.com> # for arch/riscv