dma-continuguous.h is generic for all architectures except arm32 which has
its own version.
Similar change was done for msi.h by commit 
a1b39bae16a6
("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
 generic-y += delay.h
 generic-y += div64.h
 generic-y += dma.h
-generic-y += dma-contiguous.h
 generic-y += dma-mapping.h
 generic-y += early_ioremap.h
 generic-y += emergency-restart.h
 
 generic-y += device.h
 generic-y += div64.h
 generic-y += dma.h
-generic-y += dma-contiguous.h
 generic-y += dma-mapping.h
 generic-y += emergency-restart.h
 generic-y += exec.h
 
 generated-y += syscall_table_64_o32.h
 generic-y += current.h
 generic-y += device.h
-generic-y += dma-contiguous.h
 generic-y += emergency-restart.h
 generic-y += export.h
 generic-y += irq_work.h
 
 generic-y += extable.h
 generic-y += flat.h
 generic-y += dma.h
-generic-y += dma-contiguous.h
 generic-y += dma-mapping.h
 generic-y += emergency-restart.h
 generic-y += exec.h
 
 generic-y += asm-offsets.h
 generic-y += cacheflush.h
 generic-y += device.h
-generic-y += dma-contiguous.h
 generic-y += dma-mapping.h
 generic-y += div64.h
 generic-y += emergency-restart.h
 
 generated-y += unistd_64_x32.h
 generated-y += xen-hypercalls.h
 
-generic-y += dma-contiguous.h
 generic-y += early_ioremap.h
 generic-y += export.h
 generic-y += mcs_spinlock.h
 
 generic-y += compat.h
 generic-y += device.h
 generic-y += div64.h
-generic-y += dma-contiguous.h
 generic-y += dma-mapping.h
 generic-y += emergency-restart.h
 generic-y += exec.h
 
 # (This file is not included when SRCARCH=um since UML borrows several
 # asm headers from the host architecutre.)
 
+mandatory-y += dma-contiguous.h
 mandatory-y += msi.h
 mandatory-y += simd.h