sunxi: Add PSCI support for R40

The R40's CPU controls are a combination of sun6i and sun7i.

All controls are in the CPUCFG block, and it seems the R40 does not
have a PRCM block. The core reset, power gating and clamp controls
are grouped like sun6i.

Last, the R40 does not have a secure SRAM block.

This patch adds a PSCI implementation for CPU bring-up and hotplug
for the R40.

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