boards: mimxrt1060: add lpuart8 pinctrl nodes for RT1060

The lpuart8 pinctrl nodes for RT1060 need to be defined, as WIFI + BT
(bt_tester) scenario requires three uart.
- LPUART1: shell uart
- LPUART3: bt hci uart
- LPUART8: uart pipe

Signed-off-by: Gang Li <gang.li_1@nxp.com>