BCM270X_DT: Add pwm and pwm-2chan overlays

From the README entries:
  Legal pin,function combinations for each channel:
    PWM0: 12,4(Alt0) 18,2(Alt5) 40,4(Alt0)            52,5(Alt1)
    PWM1: 13,4(Alt0) 19,2(Alt5) 41,4(Alt0) 45,4(Alt0) 53,5(Alt1)
  N.B.:
    1) Pin 18 is the only one available on all platforms, and
       it is the one used by the I2S audio interface.
       Pins 12 and 13 might be better choices on an A+, B+ or Pi2.
    2) The onboard analogue audio output uses both PWM channels.
    3) So be careful mixing audio and PWM.
    4) Currently the clock must have been enabled and configured
       by other means.

See: #756

drivers/pwm/Kconfig

@@ -85,7 +85,7 @@ config PWM_BCM_KONA
 
 config PWM_BCM2835
 	tristate "BCM2835 PWM support"
-	depends on ARCH_BCM2835
+	depends on ARCH_BCM2835 || ARCH_BCM2708 || ARCH_BCM2709
 	help
 	  PWM framework driver for BCM2835 controller (Raspberry Pi)