rockchip: px30: add uart5 as option for serial output

The px30 mini-evb can use either uart2 (muxed with the sd-card pins) or
uart5 via its pin header for serial output. Uart5 is especially useful
when needing to boot from the sd-card, where uart2 obviously is not
useable.

So add the uart5 constants and it as uart option for the serial-param
handler.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Change-Id: Ib88df7a55d761ee104d312c9953a13de3beba1c4