ARM: ux500: Describe UART platform registering issues more accurately

UARTs no longer require call-back information, since the reset
call-back was removed in 43b5f0d6

.
The only AUXDATA dependencies remaining for UARTs are DMA settings.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>