pwm: lpss: make it buildable only on X86

There is no sign of this IP block on non-x86 architectures and rather will not
be. Thus, make this explicit by applying a direct dependency to X86.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
index 0907416..3865dfb 100644
--- a/drivers/pwm/Kconfig
+++ b/drivers/pwm/Kconfig
@@ -150,6 +150,7 @@
 
 config PWM_LPSS
 	tristate "Intel LPSS PWM support"
+	depends on X86
 	help
 	  Generic PWM framework driver for Intel Low Power Subsystem PWM
 	  controller.