cmd: zynq: rsa: Setup proper dependency on zynq

This function can't be called on different SoC that's why add dependency
on Zynq before this is moved to board/xilinx/zynq folder.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

cmd/Kconfig

@@ -1167,6 +1167,7 @@ config CMD_ZYNQ_AES
 
 config CMD_ZYNQ_RSA
 	bool "Zynq RSA"
+	depends on ARCH_ZYNQ
 	select RSA
 	help
 	  Verifies the authenticated and encrypted zynq images.