fpga: xilinx-pr-decoupler: Use readl wrapper instead of pure readl

fpga: xilinx-pr-decoupler: Use readl wrapper instead of pure readl

Driver has IO wrappers but xlnx_pr_decouple_read() is not used and readl is
used instead which is just wrong.
It is also generating sparse issue that xlnx_pr_decouple_read() is unused.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Tom Rix <trix@redhat.com>
Acked-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/0381e4e8061c2fee182a104768e84feff3a82d25.1679412800.git.michal.simek@amd.com
Signed-off-by: Xu Yilun <yilun.xu@intel.com>