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/mmc/zynq_sdhci.c b/drivers/mmc/zynq_sdhci.c
index d9ad0ff..b79c402 100644
--- a/drivers/mmc/zynq_sdhci.c
+++ b/drivers/mmc/zynq_sdhci.c
@@ -577,7 +577,7 @@
 	debug("%s: CLK %ld\n", __func__, clock);
 
 	ret = clk_enable(&clk);
-	if (ret && ret != -ENOSYS) {
+	if (ret) {
 		dev_err(dev, "failed to enable clock\n");
 		return ret;
 	}