sunxi: psci: Move entry address setting to separate function

Currently we set the entry address in the psci_cpu_on function.
However R40 has a different register for this. This resulted in
an #ifdef / #else block in psci_cpu_on, which we avoided having
in the first place.

Move this part into a separate function, defined differently for
the R40 as opposed to the other single cluster platforms.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>