Right now all devices supported in the driver have the single clock: it
acts simultaneously as a bus clock (providing register interface
clocking) and source clock (driving watchdog counter). Some newer Exynos
chips, like Exynos850, have two separate clocks for that. In that case
two clocks will be passed to the driver from the resource provider, e.g.
Device Tree. Provide necessary infrastructure to support that case:
  - use source clock's rate for all timer related calculations
  - use bus clock to gate/ungate the register interface

All devices that use the single clock are kept intact: if only one clock
is passed from Device Tree, it will be used for both purposes as before.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
---
Changes in v2:
  - Reworded commit message to be more formal
  - Used separate "has_src_clk" trait to tell if source clock is present
  - Renamed clock variables to match their purpose
  - Removed caching source clock rate, obtaining it in place each time instead
  - Renamed err labels for more consistency

 drivers/watchdog/s3c2410_wdt.c | 52 +++++++++++++++++++++++++---------
 1 file changed, 39 insertions(+), 13 deletions(-)

diff --git a/drivers/watchdog/s3c2410_wdt.c b/drivers/watchdog/s3c2410_wdt.c
index fdb1a1e9bd04..c733917c5470 100644
--- a/drivers/watchdog/s3c2410_wdt.c
+++ b/drivers/watchdog/s3c2410_wdt.c

@@ -118,7 +118,9 @@ struct s3c2410_wdt_variant {
 
 struct s3c2410_wdt {
 	struct device		*dev;
-	struct clk		*clock;
+	struct clk		*bus_clk; /* for register interface (PCLK) */
+	struct clk		*src_clk; /* for WDT counter */
+	bool			has_src_clk;