The default behaviour of the uart-rx pins on the rk3188 is to be pulled up and
a lot of designs use diodes to even prevent them from being raised from the
outside.

Therefore change the rx-pin settings accordingly.

This also fixes a uart receive problem on mass production Radxa Rock boards.
Signed-off-by: Max Schwarz <max.schwarz@online.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>