refactor(allwinner): patch the DTB after setting up PSCI

Idle states are advertised to the rich OS by declaring them in the DTB.
Since the availability of idle states depends on which PSCI
implementation was chosen, the DTB must be updated after PSCI setup.

Move this operation to bl31_plat_runtime_setup, the platform hook
which happens at the right time. Defining this hook overrides the weak
definition from plat/common, so copy over the code from there, too.

Change-Id: I42a83edb9cb28e1803d17dc2d73dbc879d885222
Signed-off-by: Samuel Holland <samuel@sholland.org>