ARM: debug-ll: rework gemini handling

Gemini can not yet be configured in a multiplatform kernel, but
if we ever get there, enabling one of the gemini platforms
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 gemini default addresses and
the flow control settings only if we have also picked the respective
Kconfig symbols introduced here.

While we're at it, this also reorders the virtual address as
it should be.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Hans Ulli Kroll <ulli.kroll@googlemail.com>