clk: Add PWM clock driver

Some board designers, when running out of clock output pads, decide to
(mis)use PWM output pads to provide a clock to external components.
This driver supports this practice by providing an adapter between the
PWM and clock bindings in the device tree. As the PWM bindings specify
the period in the device tree, this is a fixed clock.

Tested-by: Janusz Uzycki <j.uzycki@elproma.com.pl>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Michael Turquette <mturquette@linaro.org>

drivers/clk/Kconfig

@@ -130,6 +130,13 @@ config COMMON_CLK_PALMAS
 	  This driver supports TI Palmas devices 32KHz output KG and KG_AUDIO
 	  using common clock framework.
 
+config COMMON_CLK_PWM
+	tristate "Clock driver for PWMs used as clock outputs"
+	depends on PWM
+	---help---
+	  Adapter driver so that any PWM output can be (mis)used as clock signal
+	  at 50% duty cycle.
+
 config COMMON_CLK_PXA
 	def_bool COMMON_CLK && ARCH_PXA
 	---help---