drivers: ipm: xlnx: fix AMD copyright

There is nothing like AMD-Xilinx Inc. That's why use full AMD name
instead.

Fixes: 09e2a4e9 ("drivers: ipm: add zynqmp r5f support")
Co-developed-by: Mubin Sayyed <mubin.sayyed@amd.com>
Signed-off-by: Mubin Sayyed <mubin.sayyed@amd.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>