ARM: ux500: simplify and move debug UART

The debug UART code defined three different virtual addresses
while only one is ever used. Get rid of this, and move the
UART remapping to 0xfffe8000 where DTCM reside on some platforms
but not on Ux500, so it can be reused moving the UART out of the
vmalloc area.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>