powerpc/mm/drmem: Silence drmem_init() early return

It's not an error or noteworthy condition if the
"ibm,dynamic-reconfiguration-memory" node isn't present.

Drop the needless message.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240603-silence-drmem_init-v1-1-e9d71646bc3d@linux.ibm.com