arm64: dts: rockchip: Remove clock-names from PWM nodes

A test with the command below gives this error:

/arch/arm64/boot/dts/rockchip/rk3368-evb-act8846.dt.yaml:
pwm@ff680030: clock-names: ['pwm'] is too short

Devices with only one PWM clock use it to both to derive the functional
clock for the device and as the bus clock. The driver does not need
"clock-names" to get a handle, so remove them all.

make ARCH=arm64 dtbs_check
DT_SCHEMA_FILES=Documentation/devicetree/bindings/pwm/pwm-rockchip.yaml

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>