arm64: dts: add all hi6220 uart nodes

This patch adds all UART nodes for the Hi6220 SoC. Recently a board[1] has
been developed to standardize UART access across all the 96boards consumer
edition boards. To use this hardware on HiKey we must configure and enable
UART3. However, to ensure backward compatibility we must keep UART0 enabled
as well.

I have removed the hard coded clock index values in favor of using the ones
already defined in include/dt-bindings/clock/hi6220-clock.h.

Since UART0 needs to be soldered, it has been suggested to use the UART3 as
the default console.

This patch was boot tested on top of next-20150930, with both UART
configurations.

[1] http://www.seeedstudio.com/depot/96Boards-UART-p-2525.html?ref=newInBazaar



Signed-off-by: Tyler Baker <tyler.baker@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>