of: Handle memory@0 node on PPC32 only

In order to deal with an firmware bug on a specific ppc32 platform
(longtrail), early_init_dt_scan_memory() looks for a node called
memory@0 on all platforms. Restrict this quirk to ppc32 kernels only.

Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org