powerpc/4xx: Add __init attribute to eligible functions

Some functions defined in 'arch/powerpc/platforms/4xx' are deserving of an
`__init` macro attribute. These functions are only called by other
initialization functions and therefore should inherit the attribute.

Signed-off-by: Nick Child <nick.child@ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211216220035.605465-15-nick.child@ibm.com