powerpc/powermac: Fix symbol not declared warnings

1. ppc_override_l2cr and ppc_override_l2cr_value are only used in
   l2cr_init() function, remove them and used *l2cr directly.
2. has_l2cache is not used outside of the file, so mark it static and
   do not initialise statics to 0.

Fixes the following warnings:

  arch/powerpc/platforms/powermac/setup.c:73:5: warning: symbol
  'ppc_override_l2cr' was not declared. Should it be static?
  arch/powerpc/platforms/powermac/setup.c:74:5: warning: symbol
  'ppc_override_l2cr_value' was not declared. Should it be static?
  arch/powerpc/platforms/powermac/setup.c:75:5: warning: symbol
  'has_l2cache' was not declared. Should it be static?

Signed-off-by: Chen Lifu <chenlifu@huawei.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Unwrap printk string]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221103070122.340773-1-chenlifu@huawei.com