powerpc: Move free_initmem to common code

The free_initmem function is basically duplicated in mm/init_32,
and init_64, and is moved to the common 32/64-bit mm/mem.c.

All other sections except init were removed in v2.6.15 by
6c45ab99 (powerpc: Remove section
free() and linker script bits), and therefore the bulk of the executed
code is identical.

This patch also removes updating ppc_md.progress to NULL in the powermac
late_initcall.

Suggested-by: Milton Miller <miltonm@bga.com>
Suggested-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Dave Carroll <dcarroll@astekcorp.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>