arm64: rockchip: Force CONFIG_PM on Rockchip systems

A number of the Rockchip-specific drivers (IOMMU, display controllers)
are now assuming that CONFIG_PM is set, and may completely misbehave
if that's not the case.

Since there is hardly any reason for this configuration option not
to be selected anyway, let's require it (in the same way Tegra already
does).

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Olof Johansson <olof@lixom.net>