ARM: debug: Update Samsung UART config options

In a multiplatform config, the low level debug option shows several
UART port entries. Improve the user visible string so that it becomes
clear to the user about Samsung UART ports.
While at it also remove some lines from the help text that are no
longer applicable across all Samsung platforms.

Signed-off-by: Sachin Kamat <sachin.kamat@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>