clk: zynq: Add dummy clock enable function

A lot of Xilinx drivers are checking -ENOSYS which means that clock driver
doesn't have enable function. Remove this checking from drivers and create
dummy enable function as was done for clk_fixed_rate driver by
commit 6bf6d81c1112 ("clk: fixed_rate: add dummy enable() function").

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/drivers/spi/zynq_spi.c b/drivers/spi/zynq_spi.c
index 2971e55..650d4d7 100644
--- a/drivers/spi/zynq_spi.c
+++ b/drivers/spi/zynq_spi.c
@@ -143,7 +143,7 @@
 	}
 
 	ret = clk_enable(&clk);
-	if (ret && ret != -ENOSYS) {
+	if (ret) {
 		dev_err(bus, "failed to enable clock\n");
 		return ret;
 	}