m68k: sun3/3x: Add and use "sun3.h"

When building with W=1:

    arch/m68k/sun3/idprom.c:86:6: warning: no previous prototype for ‘sun3_get_model’ [-Wmissing-prototypes]
       86 | void sun3_get_model(char *model)
	  |      ^~~~~~~~~~~~~~
    arch/m68k/sun3/config.c:53:24: warning: no previous prototype for ‘sun3_init’ [-Wmissing-prototypes]
       53 | asmlinkage void __init sun3_init(void)
	  |                        ^~~~~~~~~
    arch/m68k/sun3/mmu_emu.c:117:6: warning: no previous prototype for ‘print_pte_vaddr’ [-Wmissing-prototypes]
      117 | void print_pte_vaddr (unsigned long vaddr)
	  |      ^~~~~~~~~~~~~~~
    arch/m68k/sun3/mmu_emu.c:126:13: warning: no previous prototype for ‘mmu_emu_init’ [-Wmissing-prototypes]
      126 | void __init mmu_emu_init(unsigned long bootmem_end)
	  |             ^~~~~~~~~~~~
    arch/m68k/sun3/mmu_emu.c:353:5: warning: no previous prototype for ‘mmu_emu_handle_fault’ [-Wmissing-prototypes]
      353 | int mmu_emu_handle_fault (unsigned long vaddr, int read_flag, int kernel_fault)
	  |     ^~~~~~~~~~~~~~~~~~~~
    arch/m68k/sun3/leds.c:6:6: warning: no previous prototype for ‘sun3_leds’ [-Wmissing-prototypes]
	6 | void sun3_leds(unsigned char byte)
	  |      ^~~~~~~~~
    arch/m68k/sun3/intersil.c:27:5: warning: no previous prototype for ‘sun3_hwclk’ [-Wmissing-prototypes]
       27 | int sun3_hwclk(int set, struct rtc_time *t)
	  |     ^~~~~~~~~~
    arch/m68k/sun3x/config.c:30:6: warning: no previous prototype for ‘sun3_leds’ [-Wmissing-prototypes]
       30 | void sun3_leds(unsigned char byte)
	  |      ^~~~~~~~~

Fix this by introducing a new header file "sun3.h" for holding the
prototypes of functions implemented in arch/m68k/sun3/ and
arch/m68k/sun3x/.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/87856ef9ef8955f459fb691faca921c0a688bc80.1694613528.git.geert@linux-m68k.org