MIPS: ralink: mt7628a.dtsi: Add pinctrl DT properties to the UART nodes

Now that pinmux is available, let's use it for the UART DT nodes.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Paul Burton <paul.burton@mips.com>
Cc: Harvey Hunt <harveyhuntnexus@gmail.com>
Cc: John Crispin <john@phrozen.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org