ARM: hikey: hi6220: Migrate over to DM_SERIAL and use UART3 by default.

Use DM for the pl01x serial driver on hikey. Also allow UART0 or
UART3 to be chosen via Kconfig.

By default we now output to UART3 as the latest version of ATF outputs
to this UART. Also UART3 comes out on the LS connector, as opposed to
UART0 which goes to a unpopulated header.

As part of this change we also enable CONFIG_BOARD_EARLY_INIT_F and
call the pinmux configuration code for the UART. Before we were
relying on ATF having already configured the pin configuration.

NB: Upstream Linux kernel doesn't yet support UART3, so serial console
will still be output on UART0 when booting a upstream kernel.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>