ARM: nomadik: selectively enable UART0 on boards

The S8815 board is using RX/TX on UART0, and the NHK8815 is
using RX/TX and CTS/RTS (the latter connected to a Bluetooth
chip). Activate the right groups with the u0 UART0 function
on each board and undisable it. Get rid of the old erroneous
default definition from the SoC file.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>