Found kernel won't be loaded when UART does not connect to the board. This is
because UART received one data with frame error in this case, and stop in
u-boot console.
The root cause is we set wrong pad setting for UART. The pad should be set
to pull up not pull down. The pull down will cause problem to UART START bit.

This patch fix the UART pad to 0x600020 (input and output, high drive strength,
and pull up).

Signed-off-by: Ye Li <ye.li@nxp.com>
Acked-by: Fugang Duan <fugang.duan@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>