powerpc: Move DMA64_PROPNAME define to a header

Avoid redefining the same value in multiple source.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230817162411.429-1-msuchanek@suse.de