arm64: dts: rockchip: add PCIe for M.2 M-key to rock-5b

The Radxa Rock 5B has PCIe 3x4 routed to its M.2 M-key connector
on the board's back.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20230918141451.131247-3-sebastian.reichel@collabora.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>