powerpc: Fix section mismatch warning

Add __init to MMU_setup() which uses __initdata boot_command_line.
Also MMU_setup() is only called from MMU_init(), which is also __init.

Warning appeared since commit 3e47d147.

Fixes: 3e47d147 ("powerpc: Remove powerpc specific cmd_line")
Signed-off-by: Fabian Frederick <fabf@skynet.be>
[mpe: Update changelog]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>