ARM: dts: sun8i: a83t: Add nodes for UART2-UART4

A83T has 5 UART interfaces, but only the first two have their nodes
defined in sun8i-a83t.dtsi. Add nodes for the missing interfaces.

Signed-off-by: Ondrej Jirman <megous@megous.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>