ARM: debug-ll: rework SPEAr handling

Enabling one of the SPEAr platforms in a multiplatform kernel
while trying to use DEBUG_LL for another platform can default to
the wrong UART address, as the options are purely based on the
architecture being enabled or not.

This changes the logic to use the SPEAr default addresses only
if we have also picked the respective Kconfig symbols introduced
here.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>