m68k: apollo: Make local reset, serial, and irq functions static

When building with W=1:

    arch/m68k/apollo/config.c:132:6: warning: no previous prototype for ‘dn_serial_print’ [-Wmissing-prototypes]
      110 | void dn_serial_print (const char *str)
	  |      ^~~~~~~~~~~~~~~
    arch/m68k/apollo/dn_ints.c:8:14: warning: no previous prototype for ‘apollo_irq_startup’ [-Wmissing-prototypes]
	8 | unsigned int apollo_irq_startup(struct irq_data *data)
	  |              ^~~~~~~~~~~~~~~~~~
    arch/m68k/apollo/dn_ints.c:19:6: warning: no previous prototype for ‘apollo_irq_shutdown’ [-Wmissing-prototypes]
       19 | void apollo_irq_shutdown(struct irq_data *data)
	  |      ^~~~~~~~~~~~~~~~~~~
    arch/m68k/apollo/dn_ints.c:29:6: warning: no previous prototype for ‘apollo_irq_eoi’ [-Wmissing-prototypes]
       29 | void apollo_irq_eoi(struct irq_data *data)
	  |      ^~~~~~~~~~~~~~

Fix this by making all local functions static.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/d2b3a6785d3e80069fea85921c72cf0b7afb8493.1694613528.git.geert@linux-m68k.org