platform_get_irq() prints an error message when the interrupt
is not available. So on platforms where bark interrupt is
not specified, following error message is observed on SDM845.

[    2.975888] qcom_wdt 17980000.watchdog: IRQ index 0 not found

This is also seen on SC7180, SM8150 SoCs as well.
Fix this by using platform_get_irq_optional() instead.

Fixes: 36375491 ("watchdog: qcom: support pre-timeout when the bark irq is available")
Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20191213064934.4112-1-saiprakash.ranjan@codeaurora.orgSigned-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>