arm64: dts: allwinner: a64: Add PWM controllers

The Allwinner A64 SoC features two PWM controllers, which are fully
compatible to the one used in the A13 and H3 chips.

Add the nodes for the devices (one for the "normal" PWM, the other for
the one in the CPUS domain) and the pins their outputs are connected to.

On the A64 the "normal" PWM is muxed together with one of the MDIO pins
used to communicate with the Ethernet PHY, so it won't be usable on many
boards. But the Pinebook laptop uses this pin for controlling the LCD
backlight.

On Pine64 the CPUS PWM pin however is routed to the "RPi2" header,
at the same location as the PWM pin on the RaspberryPi.

Tested on Pinebook and Teres-I

[vasily: fixed comment message as requested by Stefan Bruens, added default
         muxing options to pwm and r_pwm nodes]

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Tested-by: Harald Geyer <harald@ccbib.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>