[POWERPC] Add of_platform_bus support to mpc52xx psc uart driver

Needed to support mpc52xx boards in arch/powerpc.  This patch
retains the platform_bus support when compiling for arch/ppc,
but uses the of_platform bindings for arch/powerpc.

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>