fpga: Guard the LOADMK functionality with CMD_FPGA_LOADMK

Guard the LOADMK functionality with config to provide
an option to enable or disable it.
Enable it for all platforms in mainline which enable CONFIG_CMD_FPGA.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/README b/README
index 5f89552..d362880 100644
--- a/README
+++ b/README
@@ -2550,6 +2550,10 @@
 
 		Specify the number of FPGA devices to support.
 
+		CONFIG_CMD_FPGA_LOADMK
+
+		Enable support for fpga loadmk command
+
 		CONFIG_SYS_FPGA_PROG_FEEDBACK
 
 		Enable printing of hash marks during FPGA configuration.