ARM: plat-samsung: use Kconfig choice for debug UART selection

Now that the DEBUG_LL UART can be selected by a Kconfig choice, convert
the Samsung UART selection to use a set of bools rather than an int.

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