sparc64: Fix prototype warning for dma_4v_iotsb_bind

Fix the following warning:
sparc/kernel/pci_sun4v.c:259:15: warning: no previous prototype for ‘dma_4v_iotsb_bind’

The function dma_4v_iotsb_bind is not used outside the file, so declare
it static.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Andreas Larsson <andreas@gaisler.com>
Cc: David S. Miller <davem@davemloft.net>
Reviewed-by: Andreas Larsson <andreas@gaisler.com>
Tested-by: Andreas Larsson <andreas@gaisler.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20240330-sparc64-warnings-v1-6-37201023ee2f@ravnborg.org
Signed-off-by: Andreas Larsson <andreas@gaisler.com>