powerpc/8xx: Remove init_internal_rtc() to fix no previous prototype error

A W=1 build of mpc885_ads_defconfig throws the following error:

  CC      arch/powerpc/platforms/8xx/m8xx_setup.o
arch/powerpc/platforms/8xx/m8xx_setup.c:41:1: error: no previous prototype for 'init_internal_rtc' [-Werror=missing-prototypes]
   41 | init_internal_rtc(void)
      | ^~~~~~~~~~~~~~~~~

init_internal_rtc() was introduced by commit df34403d ("[POWERPC]
8xx: Add mpc885ads support and common mpc8xx files") as a weak
function but has never been defined and/or used outside m8xx_setup.c

As it is called only once there, just fold it into its caller and
remove it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/0aa1141e18a84d926e199093204b37ec993f0c87.1692275185.git.christophe.leroy@csgroup.eu