ARM: debug: clean up low level kernel debugging selection

It is silly to bury UART selection under multiple levels of choice
statements, where the top level choice statement will only list
about four entries when a single SoC is selected.  Move the UART
selection up into the top level choice statement as it was always
intended to be.
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>