rockchip: dts: rk3399: Sync pwm2_pin_pull_down from Linux 5.1-rc2

To make successful build with dts(i) files syncing from Linux 5.1-rc2
the rk3399.dtsi would require pwm2_pin_pull_down.

So, sync the pwm2_pin_pull_down node from Linux 5.1-rc2.  Since this
node is strictly not part of any commit alone, I have mentioned
Linux 5.1-rc2 tag for future reference of where would this sync
coming from.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>