m68k: sun3: Improve Sun3/3x DVMA abstraction in <asm/dvma.h>

m68k: sun3: Improve Sun3/3x DVMA abstraction in <asm/dvma.h>

When building with W=1:

    arch/m68k/sun3/dvma.c:65:13: warning: no previous prototype for ‘sun3_dvma_init’ [-Wmissing-prototypes]
       65 | void __init sun3_dvma_init(void)
	  |             ^~~~~~~~~~~~~~

    arch/m68k/sun3x/dvma.c:178:6: warning: no previous prototype for ‘dvma_unmap_iommu’ [-Wmissing-prototypes]
      179 | void dvma_unmap_iommu(unsigned long baddr, int len)
	  |      ^~~~~~~~~~~~~~~~

Fix this by moving the declarations for sun3_dvma_init() and
dvma_unmap_iommu() to <asm/dvma.h>.  Avoid #ifdefs in callers by
providing dummy static inline functions.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/ec93ed1b28b75674ae9a0b42de083a9bb140f0b3.1694613528.git.geert@linux-m68k.org