arm64: dts: allwinner: pine64: Add remaining UART aliases

Enabling uart2 node currently leads to a /dev/ttyS1 device, with ttyS0..4
always present, causing confusion on the user's part.

dtc cannot resolve an overlay's &uart2 reference for strings, only for
phandles, so it would need to hardcode the full node path.

Avoid this and enforce reliable numbering by adding serialX aliases for:

UART1 - on Wifi/BT connector
UART2 - on Pi-2 connector
UART3 - on Euler connector
UART4 - on Euler connector

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>