ARM: realview: use Kconfig choice for debug UART selection

Now that the DEBUG_LL UART can be selected by a Kconfig choice, simplify
the #ifdefery in debug-macro.S and add entries to the top-level
Kconfig.debug instead.

Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>