[POWERPC] hvc_rtas_init() must be __init

This fixes the following section mismatch:

<--  snip  -->

...
WARNING: vmlinux.o(.text+0x2fbca8): Section mismatch in reference from the function .hvc_rtas_init() to the function .devinit.text:.hvc_alloc()
...

<--  snip  -->

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>