soc: xilinx: Move power-domain driver to the genpd dir

To simplify with maintenance let's move the xilinx power-domain driver to
the new genpd directory. Going forward, patches are intended to be managed
through a separate git tree, according to MAINTAINERS.

Cc: Michal Simek <michal.simek@amd.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>