[POWERPC] Add common routines for 52xx support in arch/powerpc

Adds utility routines used by 52xx device drivers and board support
code.  Main functionality is to add device nodes to the of_platform_bus,
retrieve the IPB bus frequency, and find+ioremap device registers.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>