ARM: debug-ll: rework integrator/versatile handling

Enabling one of the integrator 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 integrator default addresses only
if we have also picked the respective Kconfig symbols introduced
here. Versatile is not yet part of multiplatform, but hopefully
soon will be, so we do the same change for versatile as well.

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