staging: wilc1000: look for rtc_clk clock in spi mode

If rtc_clk is provided from DT, use it and enable it.
This is optional.
The signal may be hardcoded and no need to be requested,
but if DT provides it, use it.
Signed-off-by: Adham Abozaeid <adham.abozaeid@microchip.com>
Link: https://lore.kernel.org/r/20190916193701.20755-1-adham.abozaeid@microchip.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/wilc1000/wilc_spi.c

@@ -4,6 +4,7 @@
  * All rights reserved.
  */
 
+#include <linux/clk.h>
 #include <linux/spi/spi.h>
 
 #include "wilc_wfi_netdevice.h"
@@ -132,6 +133,12 @@ static int wilc_bus_probe(struct spi_device *spi)
 	wilc->bus_data = spi_priv;
 	wilc->gpio_irq = gpio;
 
+	wilc->rtc_clk = devm_clk_get(&spi->dev, "rtc_clk");
+	if (PTR_ERR_OR_ZERO(wilc->rtc_clk) == -EPROBE_DEFER)
+		return -EPROBE_DEFER;
+	else if (!IS_ERR(wilc->rtc_clk))
+		clk_prepare_enable(wilc->rtc_clk);
+
 	return 0;
 }
 
@@ -142,6 +149,10 @@ static int wilc_bus_remove(struct spi_device *spi)
 	/* free the GPIO in module remove */
 	if (wilc->gpio_irq)
 		gpiod_put(wilc->gpio_irq);
+
+	if (!IS_ERR(wilc->rtc_clk))
+		clk_disable_unprepare(wilc->rtc_clk);
+
 	wilc_netdev_cleanup(wilc);
 	return 0;
 }