boards: mimxrt1050_evk: Enable lpuart3 for bluetooth hci

Enables instance 3 of the lpuart driver, adds dts fixups, and configures
pinmuxes to use bluetooth hci on the mimxrt1050_evk board. Updates board
documentation accordingly.

Tested with samples/bluetooth/peripheral_hr, with frdm_kw41z attached as
an arduino shield running the nxp hci controller application.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>