powerpc/pseries: make pseries_devicetree_update() static

pseries_devicetree_update() has only one call site, in the same file in
which it is defined. Make it static.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220207221247.354454-1-nathanl@linux.ibm.com