ARM: sunxi: select MFD_SUN6I_PRCM when sun6i arch support is enabled

Select the MFD_SUN6I_PRCM option when sun6i arch is enabled in order to get
the PRCM (Power/Reset/Clock Management) related drivers compiled.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Documentation: dt: bindings: Document ARM PSCI enable method

arm,psci is also a valid enable-method for the CPUs on ARM. Document it.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
ARM: sun6i: Use CPU_METHOD_OF_DECLARE

CPU_METHOD_OF_DECLARE allows to bind the smp_ops to a set of cpus through the
enable-method property, instead of relying on the machine to define it. Switch
to it to get closer to an empty machine.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Documentation: dt: bindings: Document Allwinner A31 enable method

Document the necently introduced A31 enable-method as a valid option.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
ARM: sun6i: Retire the smp field in A31 machine

Now that we can rely on the enable-method, remove the smp field declaration
from A31 machine.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>