arm64: dts: rockchip: change clkreq mode for rk3399-firefly

pcie_clkreqn actually doesn't work at all, so replace it with
pcie_clkreqn_cpm.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>