sh: nommu: Add missing #include <asm/cacheflush.h>

arch/sh/mm/nommu.c:76:13: warning: no previous prototype for 'kmap_coherent_init' [-Wmissing-prototypes]
arch/sh/mm/nommu.c:80:7: warning: no previous prototype for 'kmap_coherent' [-Wmissing-prototypes]
arch/sh/mm/nommu.c:86:6: warning: no previous prototype for 'kunmap_coherent' [-Wmissing-prototypes]

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/d4b2a43fcedddee3d27cfd87ff2e0bf511588aa0.1709326528.git.geert+renesas@glider.be


Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>