The GEM driver should not need to know about Zynq specific details of
RCLK related registers and bitfields in the SLCR. Move those details to
the slcr driver.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>