ARM: dts: sun8i: tbs-a711: Enable UART2 (for NEO-6M GPS module)

TBS A711 tablet contains u-blox NEO-6M module connected to UART2.
Enable UART2 to gain access to the module from userspace.

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