powerpc/5200: dts: fix memory node unit name

Fixes build warnings:
Warning (unit_address_vs_reg): /memory: node has a reg or ranges property, but no unit name

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211013220532.24759-4-agust@denx.de