[POWERPC] Fix powermac late initcall to only run on powermac

Current ppc64_defconfig kernel fails to boot on iSeries, dying with:

Unable to handle kernel paging request for data at address 0x00000000
Faulting instruction address: 0xc00000000071b258
Oops: Kernel access of bad area, sig: 11 [#1]
SMP NR_CPUS=32 iSeries
<snip>
NIP [c00000000071b258] .iSeries_src_init+0x34/0x64
LR [c000000000701bb4] .kernel_init+0x1fc/0x3bc
Call Trace:
[c000000007d0be30] [0000000000008000] 0x8000 (unreliable)
[c000000007d0bea0] [c000000000701bb4] .kernel_init+0x1fc/0x3bc
[c000000007d0bf90] [c0000000000262d4] .kernel_thread+0x4c/0x68
Instruction dump:
e922cba8 3880ffff 78840420 f8010010 f821ff91 60000000 e8090000 78095fe3
4182002c e922cb58 e862cbb0 e9290140 <e8090000> f8410028 7c0903a6 e9690010
Kernel panic - not syncing: Attempted to kill init!

This happens because some powermac code unconditionally sets
ppc_md.progress to NULL.  This patch makes sure the powermac late
initcall is only run on powermac machines.

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>