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>

drivers/pwm/Kconfig

@@ -150,6 +150,7 @@ config PWM_LPC32XX
 
 config PWM_LPSS
 	tristate "Intel LPSS PWM support"
+	depends on X86
 	help
 	  Generic PWM framework driver for Intel Low Power Subsystem PWM
 	  controller.